title: Camera onPreviewFrame not called
body: <p>when using the Camera.PreviewCallback implementation the onPreviewFrame is called without problem after initializing camera and starting preview (Camera.startPrevew()). The problem is if I make a video recording using MediaRecorder onPreviewFrame does not get called any more. </p>
  
  <p>I understand that when using MediaRecorder to record video stops the Camera.PreviewCallback, but why can't it be restarted? </p>
  
  <p>I have tried resetting the camera preview callback (setPreviewCallback(callback)) and restarting startPreview, but while I have a preview there is no call to onPreviewFrame. </p>