libstagefright: add start code for HEVC codec config data

- OMXClient appends nal start code for HVCC \ AVCC data. This nal
  start code append was avoided for HEVC because QC HEVC parser
  was adding nal start code before codec config data by default.
- Change is made in QC parser to return HVCC data without adding
  any start code, so enable adding nal start code before codec
  config data from OMXClient.
- fallback to QC HEVC parser in case video is HEVC

Change-Id: I157e75b5b99180f887261fca49b7afac13a56848