FMRadio: Immediately mute AudioTrack once stopRender is called

* This is kind of a hack, to squelch any possible output from when
  stopRender is called to when AudioTrack.stop is called. This is
  because the render thread may be playing audio and not respond
  immediately to a stopRender call, resulting in garbage being written
  to the output once the FM input is shut down. (such as in the case of
  the headphone/antenna being removed).

REF: KIPPER-560
Change-Id: Id9dab17ca95a41e5a98c16fc8bc58d9e414e4efc