Added TOP_ACTIVITY field to WXApp java class.

This is required for toast and any other control/view in future that requires context on android but one doesn't need to be provided in by wxWidgets class.
Added assignment of TOP_ACTIVITY.

WXApp.TOP_ACTIVITY should be assigned each time an activity is resumed thus giving us currently running activity as context.
Added Notify method to WXCalls class.

This is method that wxNotificationMessage calls in order to display a toast.