Use inotify to update DocumentsUI.

While user is actively looking at a directory, subscribe to inotify
events and notify of content changes to trigger requeries.  Reference
count the observers, since multiple cursors are regularly open during
requeries.

Fix leaking cursors on activity rotation; crazy loader ID generation
is no longer needed.

Bug: 10999396
Change-Id: Iddeb08a056fee80c93df8499874705bcd213a1e2