Abstract the adjustment of A2DP codec parameters

Replaced hard-coded SBC-specific hack for updating some
of its parameters with an API abstraction: A2DP_AdjustCodec()
Also, added the corresponding unit tests.

Bug: 30958229
Test: manual A2DP testing, added new unit tests
Change-Id: I51a0a019d107362f9c24829408d426a5403b0a8e