Do not re-download the unused dictionaries.

Does the following
1. Uses dictionaries from the files/ directory while populating the
   entries into the pendingUpdates table. So that a download happens only
   if the metadata.json says so.
2. Delete an unusable dictionaries from the files/ directory.

Bug: 20142708
Change-Id: Ibd738793585c39735868e324b8ad682dff0eba34