[Android] Add missing unsubscribe on activity destroy (#13023)

When the activity is destroyed an event remain attached, if the application is a singleton this cause a memory leak of the activity.

Author: kvpt

Xamarin.Forms.Platform.Android/AppCompat/FormsAppCompatActivity.cs

@@ -327,7 +327,10 @@ protected override void OnDestroy()
 			PreviousActivityDestroying.Reset();
 
 			if (_application != null)
+			{
+				_application.PropertyChanging -= AppOnPropertyChanging;
 				_application.PropertyChanged -= AppOnPropertyChanged;
+			}
 
 			PopupManager.Unsubscribe(this);