AVFoundation: fix retrieving tracks information from live sources.

For live sources, tracks information is available only after the
AVPlayer changed its status to AVPlayerStatusReadyToPlay. It also
seems to be available only from AVPlayerItem.tracks rather than
AVAsset.tracks.
The audioAvailableChanged() and videoAvailableChanged()
signals are now correclty emitted and the video layer is
correctly positioned for live sources.

Task-number: QTBUG-38666
Change-Id: I8ee015a6ce81694c1fc1e44c679887cf7ccb0fd6
Reviewed-by: Andy Nichols <andy.nichols@digia.com>