Sort order is sticky for session, back leaves.

Instead of persisting sort order per-directory, the order is now
sticky for that session.  Any user selected sort order takes
precedence over an ordering hinted by a backend.

When a restored DocumentStack is untouched, the back key now leaves
the dialog, instead of popping from the restored stack.

Persist list/grid mode changes async.

Bug: 10659604, 10672973
Change-Id: I9f022a081c014537447c9c2af10e19d8cd9566aa