audio:Avoid VideoLagUpdate if there is no AudioPlayer

-When AudioTrack is failed to initialized, mAudioPlayer
 is deleted in onVideoEvent but onVideoLagUpdate() is still
 trying to access mAudioPlayer which is leading to a crash.
 The function onVideoLagUpdate() is used to notify if video
 is lagging behind audio. So this function is no longer
 valid if AudioPlayer is deleted.

-Fix is to add a check for AudioPlayer in onVideoLagUpdate,
 which will avoid onVideoLagUpdate() if there is no
 AudioPlayer

Change-Id: Ie8dbfc1995a3da88a4a435ec2de86b1c22a07c65
CRs-Fixed: 647200