Modularize BtaAvCo into smaller components.

Problem: BtaAvCo (bta_av_co.cc) source file is overloaded
with lots of responsibilities

Solution: The declaration should be defined in its own header file.
Also, the peer declaration should be in its own header file to
allow writing unit-test cases.

Bug: 326496929
Test: m seahawk-trunk-userdebug
Test: atest net_bt*
Flag: EXEMPT, no logical change, only refactor
Change-Id: I0afac438d0c240beb8e7a3fe86ed750f8e396aad