Prevent crash in AppWidgetHost that can be triggered by an AppWidget

-> When the RemoteViewsFactory violates the getViewTypeCount() contract
   we detect it, and prevent the AdapterView from crashing
-> Also made RemoteViewsCache a static inner class, since we may need
   that down the road, and there's no reason it shouldn't be

Change-Id: I872a255167aac94513e522924179de61286b995a