New default IClientCallback implementation

Add a default IClientCallback implementation which exits once all
watched services have no more clients. This is useful for lazy HALs.

Test: Use DefaultCounterExitCallback to register a HAL that implements
      two interfaces and check that it exits when both interfaces are
      unused.
Change-Id: Ifcc53631e284375a436d618e84e9546a5e53844a
3 files changed