Bug 1089718 - [music] Make the metadata parser sane

I think for the time being, we should leave this as-is, since it's how it used to be, and we'd need to write a migration step to wipe the asyncStorage (which is harder than it seems because we use it to store the player's settings too).

I do plan to improve our cover art caching though. I think it's the source of some of our startup jankiness.