audio: fix meta MIME type failure in MPEG4Writer

- MIME type info included in source->getFormat()
  is not updated properly because only temp
  variable is updated in AudioSource::getFormat().
  MIME type string contains garbage value and it
  results in invalid audio formats.
  MPEG4Writer returns ERROR_UNSUPPORTED and tunnel
  mode recording with 3gpp format fails
- Fix this issue by assigning return value of
  getFormat() to a local variable, and check the
  return value of findCString.

Change-Id: I242fa25a5d4f7b7e2c88fdd855208dcd42883b6c