AVDTP: Use the same MTU in config request

MTU in config request indicates how many bytes _we_ can process at most.
Instead of using different number for signalling and audio channel, we
use the same number, which is the greater one . This won't require the
remote side to have a larger buffer size, as they can still send the
same packet size as before. We won't need to worry about memory
implication either, because audio buffer will go away quickly.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: Iab025784699a77d89d784926d1f393a3cc2fe767