Resolve unmapped document when the device is connected.

Once MTP device is disconnected from Android, the files on MTP device
are marked as 'DISCONNECTED' in metadata database. These metadata will
be back when MtpDocumentsProvider finds the reconnected MTP device and
fetches the files again.

Previously the 'DISCONNECTED' files are not automatically
refetched. User needs to see files in Documents UI again to reuse
document ID of 'DISCONNECTED' files. The CL changes DocumentLoader so
that it automatically refetches disconnected documents.

BUG=26212981

Change-Id: I5cb2cc9c11af72632e481c59a505794f43ed62ea