Fix file deletion after the move to Model IDs.

- Add methods to the DocumentsAdatper to hide and unhide files.  This
  removes that burden from the Model.

- Remove clunky markForDeletion/finalizeDeletion and all related code
  from the Model.  Replace with a straight-up delete method.

- Modify deletion code in the DirectoryFragment.  Deletion now looks
  like:
  - user presses delete
  - DocumentsAdapter hides the deleted files
  - If the user presses cancel, the DocumentsAdapter unhides the files.
  - If the user doesn't cancel, Model.delete is called to delete the
    files.

- Fix deletion-related Model tests.

BUG=26024369

Change-Id: I02e2131c1aff1ebcd0bdb93d374675fd157d7f51