Xamarin Forms (Android) Notification Push

I'm writing an app following the article of https://docs.microsoft.com/en-us/azure/app-service-mobile/app-service-mobile-xamarin-forms-get-started-push

Ok, all is perfect. My app receives notifications but when my app is closed by the SO by inactivity then my device not receive nothing.

I look many examples, but I did can't to get to run the functionality.

In my MainActivity.cs I have the next code,

static MainActivity instance = null;
    // Return the current activity instance.
    public static MainActivity CurrentActivity
    {
        get
        {
            return instance;
        }
    }


    protected override void OnCreate(Bundle bundle)
    {

        instance = this;

        TabLayoutResource = Resource.Layout.Tabbar;
        ToolbarResource = Resource.Layout.Toolbar;

        base.OnCreate(bundle);

        global::Xamarin.Forms.Forms.Init(this, bundle);
        LoadApplication(new App());

        PushRegister();
    }

    private void PushRegister()
    {

        try
        {
            // Check to ensure everything's set up right
            GcmClient.CheckDevice(this);
            GcmClient.CheckManifest(this);

            GcmClient.Register(this, PushHandlerBroadcastReceiver.SENDER_IDS);
        }
        catch (Java.Net.MalformedURLException)
        {
            CreateAndShowDialog("There was an error creating the client. Verify the URL.", "Error");
        }
        catch (Exception e)
        {
            CreateAndShowDialog(e.Message, "Error");
        }
    }

    private void CreateAndShowDialog(String message, String title)
    {
        AlertDialog.Builder builder = new AlertDialog.Builder(this);

        builder.SetMessage(message);
        builder.SetTitle(title);
        builder.Create().Show();
    }

And I have other file named GmcService.cs with the next code,

[assembly: Permission(Name = "company.samplepush.mobile.droid.permission.C2D_MESSAGE")]
[assembly: UsesPermission(Name = "company.samplepush.mobile.droid.permission.C2D_MESSAGE")]
[assembly: UsesPermission(Name = "com.google.android.c2dm.permission.RECEIVE")]
[assembly: UsesPermission(Name = "android.permission.INTERNET")]
[assembly: UsesPermission(Name = "android.permission.WAKE_LOCK")]
[assembly: UsesPermission(Name = "android.permission.RECEIVE_BOOT_COMPLETED")]
namespace Adico.SamplePush.Mobile.Droid
{

[Service]
public class GcmService : GcmServiceBase
{
    public static string RegistrationID { get; private set; }

    public GcmService() : base(PushHandlerBroadcastReceiver.SENDER_IDS) { }

    protected override void OnRegistered(Context context, string registrationId)
    {
        Log.Verbose("PushHandlerBroadcastReceiver", "GCM Registered: " + registrationId);
        RegistrationID = registrationId;

        var push = App.AzurePushService.ServiceClient.GetPush();

        MainActivity.CurrentActivity.RunOnUiThread(() => Register(push, new List<string>()));
    }

    public async void Register(Microsoft.WindowsAzure.MobileServices.Push push, IEnumerable<string> tags)
    {
        try
        {

            const string templateBodyGCM = "{\"data\":{\"message\":\"$(messageParam)\"}}";

            JObject templates = new JObject();
            templates["genericMessage"] = new JObject
            {
                { "body", templateBodyGCM }
            };

            await push.RegisterAsync(RegistrationID, templates);
            var data = App.AzurePushService.ServiceClient.InstallationId;

            Log.Info("Push Installation Id", push.InstallationId.ToString());
        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(ex.Message);
            Debugger.Break();
        }
    }

    protected override async void OnMessage(Context context, Intent intent)
    {
        Log.Info("PushHandlerBroadcastReceiver", "GCM Message Received!");

        await System.Threading.Tasks.Task.Delay(500);

        var msg = new StringBuilder();

        if (intent != null && intent.Extras != null)
        {
            foreach (var key in intent.Extras.KeySet())
                msg.AppendLine(key + "=" + intent.Extras.Get(key).ToString());
        }

        //Store the message
        var prefs = GetSharedPreferences(context.PackageName, FileCreationMode.Private);
        var edit = prefs.Edit();
        edit.PutString("last_msg", msg.ToString());
        edit.Commit();

        string message = intent.Extras.GetString("message");
        if (!string.IsNullOrEmpty(message))
        {
            createNotification("New todo item!", "Todo item: " + message);
            return;
        }


        string msg2 = intent.Extras.GetString("name");

        createNotification("Unknown message details", msg.ToString());
    }

    void createNotification(string title, string desc)
    {
        //Create notification
        var notificationManager = GetSystemService(Context.NotificationService) as NotificationManager;

        //Create an intent to show ui
        var uiIntent = new Intent(this, typeof(MainActivity));

        //Use Notification Builder
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this);

        //Create the notification
        //we use the pending intent, passing our ui intent over which will get called
        //when the notification is tapped.
        var notification = builder.SetContentIntent(PendingIntent.GetActivity(this, 0, uiIntent, 0))
                .SetSmallIcon(Android.Resource.Drawable.SymActionEmail)
                .SetTicker(title)
                .SetContentTitle(title)
                .SetContentText(desc)
                .SetSound(RingtoneManager.GetDefaultUri(RingtoneType.Notification))
                .SetAutoCancel(true).Build();

        //Show the notification
        notificationManager.Notify(1, notification);
    }

    protected async override void OnUnRegistered(Context context, string registrationId)
    {

        var push = App.AzurePushService.ServiceClient.GetPush();

        await push.UnregisterAsync();

        Log.Error("PushHandlerBroadcastReceiver", "Unregistered RegisterationId : " + registrationId);
    }

    protected override void OnError(Context context, string errorId)
    {
        Log.Error("PushHandlerBroadcastReceiver", "GCM Error: " + errorId);
    }
}

[BroadcastReceiver(Permission = Gcm.Client.Constants.PERMISSION_GCM_INTENTS)]
[IntentFilter(new string[] { Gcm.Client.Constants.INTENT_FROM_GCM_MESSAGE }, Categories = new string[] { "company.samplepush.mobile.droid" })]
[IntentFilter(new string[] { Gcm.Client.Constants.INTENT_FROM_GCM_REGISTRATION_CALLBACK }, Categories = new string[] { "company.samplepush.mobile.droid" })]
[IntentFilter(new string[] { Gcm.Client.Constants.INTENT_FROM_GCM_LIBRARY_RETRY }, Categories = new string[] { "company.samplepush.mobile.droid" })]
public class PushHandlerBroadcastReceiver : GcmBroadcastReceiverBase<GcmService>
{
    public static string[] SENDER_IDS = new string[] { "XXXXXXXXXXXXX" };

    public override void OnReceive(Context context, Intent intent)
    {
        base.OnReceive(context, intent);
    }
}

}

All run good, but when my app is closed by the SO... The push notifications aren't received.

I have spent many hours in this trouble.. please, Can you help me to get receive the notifications always? Like whatsapp and other apps...