Merge changes from topic "opus_encoder_0124"

* changes:
  libstagefright: Add support for muxing Opus files to Ogg format with unified CSD
  codec2: add C2SoftOpusEnc
  C2SoftOpusDec: Add support for decoding single CSD
  OpusHeader: Add support for unified CSD
  libstagefright: Move OpusHeader files to libstagefright_foundation