Added A2DP codec-related APIs

Those APIs can be used to check various codec-related characterictics,
and minimize the codec-specific knowledge in the rest of the code.

The added APIs are:
 - A2D_GetCodecType()
 - A2D_IsCodecSupported()
 - A2D_UsesRtpHeader()
 - A2D_IsVendorCodecSupported()
 - A2D_VendorCodecGetVendorId()
 - A2D_VendorCodecGetCodecId()
 - A2D_VendorUsesRtpHeader()

Also:
 - Added unit tests for the new API calls
 - Removed redundant definition of BTA_AV_CODEC_TYPE_IDX
 - Removed unused A2D_MEDIA_CT_* codec type defines

Bug: 30958229
Change-Id: Id9ab01fd93b0b0a9b55a0af9bfb482310ae0ade4