Handle null bindings returned from ConnectionService.

When a ConnectionService returns a null binding, immediately unbind from
the ConnectionService and cancel any ongoing calls related to it.

Bug: 211114016
Test: Added new CTS test to verify auto unbind from null binding ConnectionService.
Test: Manually tested using test app which implements null binding ConnectionService and verified via telecom log inspection that the service is unbound and the call is terminated.
Change-Id: I0757557e66725dddfd871cd9857071a8749bd7ba