FMRadio: Pause and stop AudioTrack before stopping

* According to documentation, stop will continue to play the last
  buffer, while pause immediately halts playback.
* Pause to reduce latency to stop, then flush the AudioTrack buffer
  before actually stopping in the render thread.

REF: KIPPER-560
Change-Id: I0b497cd6b95b0921f63796746c7279a4f931dbd3