title: what is good setAudioEncodingBitRate on record voice
body: <p>hello，i want to use mediaRecorder to record voice. i want to save the format is amr.</p>
  
  <pre><code>this.mediaRecorder = new MediaRecorder();
  this.mediaRecorder.setAudioChannels(1);
  this.mediaRecorder.setAudioSamplingRate(8000);
  
  this.mediaRecorder.setAudioEncodingBitRate(16);
  this.mediaRecorder.setAudioSource(MediaRecorder.AudioSource.MIC);
  this.mediaRecorder.setOutputFormat(MediaRecorder.OutputFormat.RAW_AMR);
  this.mediaRecorder.setOutputFile(this.file.getAbsolutePath());
  
  this.mediaRecorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
  </code></pre>
  
  <p>i used  this.mediaRecorder.setAudioEncodingBitRate(16), some device is ok</p>
  
  <p>mediaRecorder.setAudioEncodingBitRate(12500),somedevice is ok</p>
  
  <p>but i delete the mediaRecorder.setAudioEncodingBitRate  some device is ok</p>
  
  <p>so  my question how to get the default the AudioEncodingBitRate.
  which parameter i need to use?</p>