MediaScanner: Do not update the file path when updating records already in the database

The path should never change (media scanner treats renames as a delete and an insert)
and we want to preserve the mixed case paths in the database to avoid exposing the
lowercase squashing to MTP.

Change-Id: I79ef37e143c607242d438a4a0643d40191965b6a
Signed-off-by: Mike Lockwood <lockwood@android.com>