ViewState screenSize not set on iOS

[mtehver]

ScreenSize is set asynchronously using the main queue on iOS and as a result screen <-> map conversion functions do not work properly in methods like viewDidLoad. A workaround is to store the view size as early as possible (in layoutSubviews method), even before the GL context has been created.

A similar issue exists on Android. In onCreate method the layout has not yet been determined. On Android ViewTreeObserver.OnGlobalLayoutListener should be used.