Localize the usage of tA2D_SBC_CIE only within the SBC codec

* Added new A2DP API and use it as appropriate
 - A2D_IsValidCodec()
 - A2D_CodecTypeEquals()
 - A2D_GetMinBitpool()
 - A2D_GetMaxBitpool()
* Removed A2D_GetDefaultConfigSbc(), because it is not needed anymore.
* Removed bta_av_co_audio_get_sbc_config() and replaced it
  with code that is not SBC codec-specific:
  bta_av_co_audio_encoder_init() and bta_av_co_audio_encoder_update()
* Removed bta_av_co_get_remote_bitpool_pref() because it is
  not used/needed anymore.
* Moved the following declarations the a2d_sbc.h header file to
  the a2d_sbc.c codec-specific implementation, because they don't need
  to be exposed anymore.
  - struct tA2D_SBC_CIE
  - function A2D_BldSbcInfo()
  - function A2D_ParsSbcInfo()
* Added corresponding unit tests for the new API

Bug: 30958229
Test: A2DP tested manually. Unit tests included in the CL.
Change-Id: Ib242956c303f0ca3c5f70ebc00416a7a45e70228