L2CAP: Add a "required_remote_mtu" and store it

For AVCT_BR and BNEP, we need a larger required remote mtu than minimal
L2CAP MTU.

So far we enforce this in profile service itself, but later we can let
L2CAP module enforce this for us.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I0b4bad4049636bcdc52615c000b1fff3acde360c