[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.
Let's go with the current approach and if we notice weird behaviour we can still go further. But since the same might already happen right now it's certainly not getting worse.