audio: Check the return code of MediaWriter::start

- Return code of MediaWriter::start method is not checked
  in startRawAudioRecording. Due to this, sound recorder
  application is not receiving the error whenever there is
  an error in starting of the recording
- Fix is to check the error and return it back to
  the client accordingly

Change-Id: I15916211da7a2eb0f27fbf9d4151410a8bdfdd42
CRs-Fixed: 591788