[infomanager] only call UpdateAVInfo when really needed

Currently we´re updating the av information whenever the infomanager is polled for `MUSICPLAYER_*`. This commit makes sure we´re only updating the information when really needed.

Second commit by @fritsch makes sure we´re not running in a race condition where the ui and addon try to update the av information.

@Montellese, @topfs2, @MartijnKaijser for review and Helix approval.
I assumed the global getters are threadsafe and delaying the call of them I did not want - could be the gui thread. 

Any reason you think it should be locked?