Update the A2DP codec setup and selection mechanism

* Update bta_av_co_audio_set_codec() so it can select among
  multiple available A2DP source codecs.
* Rename A2DP_SetCodec() to A2DP_SetSourceCodec() and update
  it to use tA2DP_CODEC_SEP_INDEX as an argument to specify
  the particular codec (instead of using SBC as default).

Also:
* Move the definition of AVDT_CODEC_SIZE from bt_target.h
  to avdt_api.h and increased its value from 10 to 20
* Add missing bta_av_co_audio_sink_has_scmst() check inside
  bta_av_co_find_peer_sink_supports_codec()
* Rename (inside bta_av_co.cc): cfg -> config

Bug: 30958229
Test: unit tests and A2DP streaming
Change-Id: I49fcf5063c3b6c4060abdfb60c2db171fa1ff747