L2CAP: Classic fixed channel reference counting

* Add a mechanism to disconnect ACL after a link is idle for kClassicLinkIdleDisconnectTimeout
  seconds. The current setting is 20.
* Add Acquire() and Release() method to ClassicFixedChannel for user to
  indicate if a fixed channel should hold the current ACL link or should
  it be oportunistic
* Implemented methods to register for disconnection callbacks
* Add tests to verify fixed channel acquire/release and register
  disconnction callback methods
* Add tests to verify classic link idle timeout disconnect methods

Bug: 138261142
Test: bluetooth_test_gd
Change-Id: I78129afb1b988311c56019f87cbb3f78e9a08cc5