A2DP Offload: Fix Max AVDTP MTU for Offload

Fix the max AVDTP MTU for Offload to what
can fit in a 3DH5 packet after accounting
for AVDTP and L2CAP header.

Also:
 - Moved MAX_2MBPS_AVDTP_MTU and MAX_3MBPS_AVDTP_MTU to the common
   avdt_api.h so it can be used by the rest of the code
 - Removed previously incorrect value of BTA_AV_MAX_A2DP_MTU and
   replace it with the new (correct) value of MAX_3MBPS_AVDTP_MTU
 - Removed duplicate define of MAX_2MBPS_AVDTP_MTU and use
   the one from avdt_api.h

Bug: 78907438
Test: A2DP Streaming
Change-Id: I2c60c34c33e4164c9f7e2e7362bbb600482dec97