Simplifying memory management, use Task Keys as resource cache keys.

- Attempts to load non-topmost task thumbnails from cache
- Ensuring that we release all references to the activity from the bg loader
- Removes background loading debug flag
- Moving callbacks into their respective classes
- cleaning up some callbacks when data is loaded in the bg

Change-Id: Ibb968349d08084922d5b28e432b76a165bf20d6b