L2cap shim: Send config callback before register dequeue

We want to let the client (AVCT) know the MTU first.  They might need to
send something without MTU available.

On AVCT side, initialize MTU as minimal L2cap MTU for now.

Bug: 179071567
Test: Speaker
Tag: #stability
Change-Id: I654c5fd302096876a3b6d245d173a735468ba949