MediaScanner: fix breakage in updates

To facilitate bulk inserts, we changed the media scanner to use the file URI
instead of the audio, video or image media URIs.
However we should still use the media URIs for updating existing records

Bug: 5021925

Change-Id: I6143dc1d00350192170428a5e951b52ef9223734
Signed-off-by: Mike Lockwood <lockwood@android.com>