L2cap: Add SecurityInterface

Add SecurityInterface as a bridge between L2cap and Security Module.

LinkSecurityInterface is used by Security Manager to access some link
methods (hold/release, disconnect). Also add a way for Security Manager
to initialize a link connection.

Test: cert/run --host
Tag: #gd-refactor
Change-Id: I08380c96f2f7ce38795d9b27b17209241fb2ec24