[Android] Java.Lang.RuntimeException: Animators may only be run on Lo… (#4177)

* [Android] Java.Lang.RuntimeException: Animators may only be run on Looper threads on Battery Saver Mode

https://github.com/xamarin/Xamarin.Forms/issues/4176

* rewrite code to BeginInvokeOnMainThread
- fixes #4176

Xamarin.Forms.Platform.Android/AndroidTicker.cs

@@ -58,7 +58,10 @@ namespace Xamarin.Forms.Platform.Android
 
 		protected override void DisableTimer()
 		{
-			_val?.Cancel();
+			Device.BeginInvokeOnMainThread(new Action(() =>
+			{
+				_val?.Cancel();
+			}));
 		}
 
 		protected override void EnableTimer()