setArtworkLoader

Loads the picture for each media item, so the application does not have to watch the player.

loader runs on a background thread when the item changes, and again when its tags arrive after the item opens. A newer item cancels an older load, and the picture is cleared while the new one loads. A loader that throws or returns null shows the item's own cover, when it has one. A picture larger than the system draws it is scaled down once to that size. Null stops loading and keeps the picture shown.