L2cap: Move LinkSecurityInterface impl to LinkManager

Handle the event in LinkManager. When the proxy is deleted, if there is
still pending request in proxy itself, it will crash. Move it to
LinkManager to ensure the request won't go away.

Tag: #gd-refactor
Test: cert/run --host
Change-Id: Id6a0fcdd55e73e236d14cef36d96c42b2bc69efa