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
ef7184a1aa