libmedia: ensure mCaptureThread exit within Visualizer destructor

If mCaptureThread doesn't exit when Visualizer instance gets destroyed,
recycled member variables with random value such as danger pointer will
be accessed again and causes crash.

Thread which is created within Visualizer instance lifecycle should be
terminated before or within destructor.

CRs-Fixed: 688546
Change-Id: Ie0b12ae8a3b9735b6ecbac0353781b8358cad644
AudioTrack: When paused, return cached playback position

An offload output can be re-used between two audio tracks having
the same configuration. A timestamp query for a paused track
while the other is running would return an incorrect time.
To fix this, cache the playback position on a pause() and return
this time when requested until the track is resumed.

CRs-Fixed: 612943

Change-Id: I54b5a851ee3fa2ceadc37e3f5c6af4f37b8f404a