[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.
Well it depends on how often the info returned by GetVideoStreamInfo/GetAudioStreamInfo changes. If it can change between two subsequent calls in very fast succession (i.e. one thread being interrupted and another one doing the same calls) then we might overwrite m_videoInfo and m_audioInfo with outdated information.