L2CAP: Add mocks for fixed channel manager and classic module

* Add mocks for l2cap::classic::FixedChannelManager
  and l2cap::classic::L2capClassicModule

Bug: 138261142
Test: make with a no-op test and run it
Change-Id: I82cd5b4caa6474b154bf9d4736e32c980fb9aba3