Use the effective peer MTU when preparing the codec configuration

Added new method A2dpCodecConfig::getEffectiveMtu() and use it
to obtain the effective MTU per peer per codec.
The effective peer MTU could be smaller than the one reported
by the peer (e.g., for interoperability reasons).

Bug: 77913350
Test: Manual
Change-Id: Idd5230681ac5856fd18dd33ac48723c6ea609e16
Merged-In: Idd5230681ac5856fd18dd33ac48723c6ea609e16
(cherry picked from commit d57fcc86)