libstagefright: Fix for CTS recording testcase failure

- AMR WB record is failing through CTS because of incorrect
  sample rate and bitrate values. Fix AMR WB encode
  bitrate and sampling rate values to fix this issue.
- Back to back recording in CTS testcase
  MediaRecorderTest#testRecordingAudioInRawFormats fails.
  Add a fix to release audio source node reference on stop
  recording as well as before creating new audio source.

Change-Id: Ia4b1f2331b0b6f9c6c7d23cd40e8c5b5cec76e28
CRs-fixed: 384336