A2DP codec related fixes and cleanup

* Update bta_av_co_audio_getconfig() to select the preferred codec
* Fix a bug in selecting the A2DP codec inside
  bta_av_co_audio_set_codec()
* No need for function A2DP_ParsSbcMplHdr() to be exposed in the API
* Generalize the following APIs, so they can be used to redirect
  the call for vendor-specific codecs:
  - A2DP_CodecSepIndexStr()
  - A2DP_InitCodecConfig()
  - A2DP_SetSourceCodec()
* Change tA2DP_ENCODER_INIT_PARAMS.SamplingFreq from uint16_t to
  uint32_t so it can store larger sampling frequency.
* Rename content protection constants from BTA_AV_CP_*
  to AVDT_CP_* and move them from bta_av_co.h to avdt_api.h
* Rename some of the internal functions in a2dp_sbc.cc
* Add extra debug logging and fix some comments

Test: A2DP streaming, unit tests passing
Change-Id: I1b67ff18dc568dd859429fcd89c644799fb1438d