Attempt at fixing crash we experience in VisualElementTracker

I am confused why this weird code is even here...
It was calling into CADisplayLinkerTicker which turns off
CheckForIllegalCrossThreadCalls
I am both scared and confused...

Author: Jonathan Peppers

Xamarin.Forms.Platform.iOS/VisualElementTracker.cs

@@ -211,7 +211,7 @@ void OnUpdateNativeControl(CALayer caLayer)
 			};
 
 			if (thread)
-				CADisplayLinkTicker.Default.Invoke(update);
+                Device.BeginInvokeOnMainThread(update);
 			else
 				update();