Disable last.fm scrobbling if music is disabled in GOA

Another scrobbling related change ;) But for now this is more a call for discussion.

I would expect that if I disable "Use for Music" in GOA for my Last.fm account that also Lollypop would not use that account for scrobbling. Hence this MR checks for music_disabled. Thoughts?

Also I am not entirely sure how to handle this check. At the moment this gets checked on start, but actually I would expect that you can change this any time in Gnome Settings and Lollypop would pick this setting up on runtime. Not sure how expensive this would be to query the GOA details all the time.