A2DP: Use codec_specific_1 to report AAC bitrate mode to upper layer

This CL uses the AAC's btav_a2dp_codec_config_t.codec_specific_1 to
store its bitrate mode, and report this information to upper layer, and
upper layer would be able to use this variable to change settings, too.

There are 7 constant numbers to present those bitrate modes:
* AACENC_BR_MODE_CBR - AAC VBR is unable to use, and upper layer can use
  this value to keep the current configuration without touched.
* AACENC_BR_MODE_VBR_C - AAC VBR is supported by both source and sink,
  and we use or are going to use AAC CBR.
* AACENC_BR_MODE_VBR_1 ~ AACENC_BR_MODE_VBR_5 - Using AAC VBR with
  different quality.

Bug: 112325138
Test: check the codec status from frameworks manually
Change-Id: I2096d9569c7aa0e5c79708946ba117d3edc4f041