Remember mode and sort on per-directory basis.

Persist the last user-selected list/grid mode and sort order for
each directory.  Remembered user choice always overrides provider
hinting.

Filter out recent documents that don't match requested MIME type, and
show recents in grid mode when picking images.  Hide mode and sort
order in recents.

Add hinting flag for backend to indicate a directory would like to be
sorted by last modified.  Include explicit root in DocumentStack and
clearly mark derived fields.

Bug: 10392047, 10608506
Change-Id: I2dd3a0e4112852ebf87e7dbb08b3781c86587dcf