Introduce L2cap security shim

L2cap security shim lets GD L2cap wants to enforce security requirements
on legacy security module.

Implementing the GD security interface.

Tag: #gd-refactor
Bug: 141555841
Test: cert/run --host
Test: Pair and connect with a headset
Change-Id: I60cd99566011f3eb93ecd6b32b1f428e8d69733d