Fix audio track pause.

AudioTrack::pause() should signal the control block condition
to release threads waiting for available buffers in obtainBuffer().
Otherwise the behavior relies on the timout on the condition
or the fact that audioflinger will mix a new audio buffer while executing
the pause.

Bug 6653769.

Change-Id: I5f8f73c471fe306070f30b814f32fd4b4dc1d575