ExternalStorageProvider: SD card's name in Document UI is unreadable

The SD card's title used in Document UI is storage volume's user label.
some times, storage volume's user label may be null or something that
is not human readable, in this case, descripition will be much better
than user label..

If we could get a non-null description, use this description as title.
Otherwise, use storage volume's user label as title.

Change-Id: Ie1adc6ff39293642ebb72d45d8896a273c515205
CRs-fixed: 740100