Reducing delay in applying widget previews loaded in the background.

- Push the writing of previews to the DB to the worker thread
  to ensure that we can apply the loaded preview as soon as
  possible.
- Reducing additional bitmap allocations from cancelled tasks
  that did not re-add their bitmaps to the unused set
  (happens when the list is scrolling quickly).

Change-Id: Ib7c2cfdfcc668c215b3a74271e1165235fbdf752