AAC-Encoder

   - Introduce metadata support for parametric stereo audio object type.
     Write metadata matrix mixdown coefficient only for channel config 5 and 6.
     Modified file(s):
        libAACenc/src/aacenc_lib.cpp

   - Revise metadata encoder downmix gain and fix matrix mixdown coefficient.
     Modified file(s):
        libAACenc/src/aacenc_lib.cpp
        libAACenc/src/metadata_compressor.cpp
        libMpegTPEnc/src/tpenc_asc.cpp
        libMpegTPEnc/src/tpenc_lib.cpp
        libMpegTPEnc/src/version

Bug 9428126

Change-Id: I87f5f1a1fdddd8223187aa0f9dc733e0ecc3e7e4