Track and persist directory stacks; recents work.

Move to manual tracking of directory navigation stack so we have Uri
data to persist, instead of opaque fragment backstack.  Remember
directory stacks across launches on a per-app basis.

Start recording recently opened and created files.  Uniform Uri
parameter extraction utility methods in contract.

Change-Id: I79ed30ee10272bf7c53d339e797639c993f649bb