Bug 1089718 - [music] Make the metadata parser sane

I changed to picture.start, but I'd like to keep the flavor property for lineage information. It could come in handy if we ever need to upgrade the database. For instance, without flavor, we wouldn't be able to tell if a file had external album art or if it was a non-temporary file with unsynced album art, which would make it harder to tell mediadb to only reparse the files with unsynced album art.