[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.
It's more your part of the code (I am only a guest here), you decide: https://github.com/fritsch/xbmc/commit/f8f3cc2e7c49987f579202a105b25b25e1bdc9a7