[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.
Yes, that is right. I thought of that polling behaviour in combination with FrameMove that is the only function that can set the m_isValid to false. Therefore we are most likely in worst 1 frame wrong.

I am afraid to do the lock over the g_application call, cause this could block us much longer than I frame lasts, do you get my reasoning?