More recents work; filtering and sorting.

Update DirectoryFragment to render List<Document>, making it more
general purpose.  Feed it documents either from a backend Cursor or
after resolving fields from a recents Cursor.  Start in recents when
no persisted stack available.  Synthesize a root for recents.

Local directory filtering and sorting using predicates and
comparators, all performed on background thread.  Introduce
UriDerivativeLoader which handles ContentObserver updates while
producing a derivative work of a Cursor.

Split data model classes into separate files.

Change-Id: Idb88b4ee22c58c8e508328e678877f7e4c978533