Fix issue #17674789: Recents icon resets if persist mode is 'persistNever'

Changed TaskPersister worker thread to not delete task information on disk
for tasks that are in recents but not persistable.

Also, changed logic that writes tasks information (excluding thunmbnail)
to disk to not write information for tasks that are not persistable.

Bug: 17674789
Change-Id: Iff7c5d24677f8a75157bb6be504fc2d9d6bcc098