libstagefright: Query AudioSystem for suggested record mute duration

- AudioSource mutes a pre-defined duration (defined by kAutoRampStartUs)
  at the beginning of a recording.
- Instead, query the audio system for any ongoing playback streams
  and use its output latency to calculate the duration to mute the incoming PCM stream.
- This assumes all current playback threads will be paused once recording
  is started.
CRs-Fixed: 438149

Change-Id: Ifaf59a9c09c0ece956379e10c801cad4dacfc8f3