[OsmDroidBinding] Fix binding of OverlayManager.Get(int).

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=12558

It was erroring out with:

	obj/Debug/generated/src/OsmDroid.Views.Overlay.OverlayManager.cs(156,58):
	error CS0508: `OsmDroid.Views.Overlay.OverlayManager.Get(int)': return type must be `Java.Lang.Object' to match overridden member `Java.Util.AbstractList.Get(int)'