Eliminate memory leak in TaskPersister

Bitmaps added to TaskPersister were piling up in the queue.

- The mRecentsChanged flag was being modified without holding the
lock. There is no mRecentsChanged flag now. Everything to be
written goes into a queue.
- TaskPersister now runs until the queue is empty.
- Bitmaps being written to the same file were being added to the
end of the queue without replacing the earlier bitmap. Now we
search the queue for matching tasks and replace the bitmaps
if needed.
- Method notify() was renamed to wakeup() so IDE could find usages
quicker.
- Bitmaps that were being requested but were still in the queue
are now being fetched from the queue.

Fixes bug 16512870.

Change-Id: Idca1c712e5d2df8196e93faaf563a54405ee96bf