A2dpCodecs: encoder standalone APIs separating from codec config

Each A2DP peer has its own codec configurations, and there is a single
encoder instance used by the active peer. The interval and effective
frame size are used by the encoder, so moving those two APIs into the
encoder interface.

Bug: 195498246
Tag: #refactor
Test: 1. atest net_test_stack
      2. Switch A2DP codec manually
Change-Id: I3e97ff6c5811f620a8add88d3606cc7993851560