Removed usage of A2DP codec_type as a separate field

The value of codec_type can be extracted from the codec_info,
so it is not needed to pass it around as an argument, or keep
it as an field.

Also:
* Added new APIs and the corresponding unit tests:
  - A2D_GetPacketTimestamp()
  - A2D_BuildCodecHeader() - replacement for bta_av_sbc_bld_hdr()
  - A2D_CodecName()
* Replaced tBTA_AV_SEP.codec_type with tBTA_AV_SEP.codec_info

Bug: 30958229
Test: A2DP tested manually. Unit tests included in the CL.
Change-Id: I3bfd4750e8639d77a9938ab3c274c9d6ea859fb6