Fix isssue 2548710: Native AudioTrack resources never freed.

The problem is a bug in AudioFlinger::MixerThread::prepareTracks_l() that makes that even if the TrackHandle
is destroyed, the corresponding Track will remain active as long as frames are ready for mixing.
If the track uses shared memory (static mode) and the sound is looped, this track will play for ever.

The fix consists in removing the track from active list immediately if the track is terminated.

Change-Id: I4582aa1d981079ab79be442fb6185f5afaed5cf3