Update Xamarin NotificationHub docs, to clarify assemblies being referenced

Adds additional content to both Xamarin.Android and Xamarin.iOS NotificationHub docs, explaining that additional assemblies are required to make it easy to integrate with NotificationHubs. Also switched the Android sample to using the GoogleCloudMessagingClient Xamarin Component to reduce the number of steps required.
Erroneous iOS screenshot also updated.
NOTE: the related sample download ZIP at xamarin.com has also been updated to reflect these changes.