A2dpCodecs: apply the encoder settings via encoder_interface->encoder_init

Because the stack supports a single A2DP encoding instance which usually
needs to be initialized again after reconfig, the new codec config
should not apply to this encoder directly. It should go through
BTIF_A2DP_Source to set up the codec by one of those encoder_interface
encoder_init APIs, so the input and output format are consistent with
its configuration.

Bug: 195498246
Tag: #compatibility
Test: atest net_test_stack
      Switch A2DP codec manually
Change-Id: Ic9143b7cf91bf57df926c97fd7f3690da6227126