L2CAP: Call AclConnection.RegisterDisconnectCallback() in LinkManager

* LinkManager should get ACL disconnection callback from HCI since it
  needs to be able to free Link objects after tearing down Link states
* Previously, only Link is getting this callback, as result, there is a
  memory leak when ACL is disconnected as the Link object was never
  removed

Test: bluetooth_test_gd
Bug: 140938432
Change-Id: I28ed509cbaaa1c4917e1f632a6c35247f3ebe2ef