Update the A2DP MTU if the L2CAP MTU changes during codec reconfiguration

Previously, the MTU was saved inside the AVDTP module after the first
codec setup. However, the MTU wasn't updated after codec reconfiguration.
As a result, if a Sink device uses different (smaller) MTU for
a codec like SBC, then switching fron another codec to SBC will
result in generating SBC frames that are too large to transmit,
and there will be no audio.

Bug: b/35351216
Test: A2DP streaming and codec switch to various Sink devices.
Change-Id: Ieb3caf4ef7a30a7af31ceb2f6132807739829441