SM: Implement the LinkSecurityInterface for managing an ACL connection

Classic security needs to manage an ACL connection for the duration of the
pairing process.  This acts as a keep-a-live for the connection to finish
the pairing process.  Additionally, it gives us a signal into whether the
remote device is still connected or not.

Bug: 145638034
Test: atest bluetooth_test_gd:SecurityManagerChannel
Test: atest bluetooth_test_gd:ClassicPairing
Test: cert/run --host --test_filter=SecurityTest
Tag: #gd-refactor
Change-Id: I53ec2b12015ebb4546ec330ffddc82061ad971da