MediaScanner: No longer delete playlist files that have no valid entries

When copying files from MTP we must keep the file in the database, even
if we cannot find any of the playlist entries (or even if it is not a valid playlist file)

BUG: 3260341

Change-Id: I4b39027ee25ffae6778224cf74830a4b1ca3fecb
Signed-off-by: Mike Lockwood <lockwood@android.com>