Replace blank artist/album names with "unknown".

When inserting something into the database and not providing an album
or artist name, -1 would be used as the album and artist id. By
substituting MediaStore.UNKNOWN_STRING, we ensure this is handled
consistently with the way the media scanner does it.
b/3371305

Change-Id: Ia292d2c8c095e4cf9c5999e5ba4d3c893cd90989
1 file changed