Re-enable sorting in the DirectoryFragment.

- Move sorting from the back-end (using SortingCursorWrapper) to the the
  front-end (in DocumentsAdapter).  This makes it such that re-sorting
  the directory contents doesn't necessitate a reload.
- Update DirectoryLoaders to just return unsorted results, and rely on
  the UI to sort them.
- Remove the (now-unused) SortingCursorWrapper.
- Update Model tests to test sorting.

BUG=26024369

Change-Id: I871cc0e496267d381ae546e0309125d04649415a