Re-architecting RemoteViewsAdapter internals due to new constraints.

- Respecting the AdapterView/Adapter contract using FrameLayout notification to update individual items
- Initial changes to allow for keeping (currently bitmap only) memory in check when loading numerous RemoteViews
- Fixing issue with leaking activities due to extra service connection references
- Fixing issue with multiple RemoteViewsAdapters being set on the same AdapterView
- Fixing small issue with StackView sometimes requesting indices out of bounds
- Removing background loading, reverting to previous message-queuing method (seems to be performant now)

Change-Id: I42313767aa791dfe35c247c97ae5d64389e6bb4c