StreamingService: Move coverart notification to it's own method.

Move the coverart stuff from showNotification() to it's own method. Also move
the MetadataEditor to the proper method, setMusicInfo(); this clears up a TODO
and should make things a little more clear for readability.
StreamingService: Remove a couple of nested blocks.

Removing large nested blocks improves readability.
Unused import and field removals.