Reference docs by ROOT_ID and DOC_ID; recents.

The same document may be present with different sematics under
multiple storage roots, so always reference using both ROOT_ID and
DOC_ID.  This enables backends to revoke permissions for an entire
root, such as when an account is removed.

Start building provider to remember recently accessed documents.

Change-Id: I75befa2e61393dec12fcc7fd27f631fcddae46fa