Update AppWidgetHost with better support for OnClickHandlers

This updates AppWidgetHost and AppWidgetHostView to do a better
job at refreshing widgets and their host views.  It now allows
an OnClickHandler to be specified when creating the AppWidgetHost
which allows it to correctly update AppWidgetHostViews when needed.

Change-Id: I710c1d00a8d145bf3a9fd5f5691885bec9d1c7e4