Ensure null ConnectionService references don't crash phone.

- Modified Call logic to remove precondition asserts for mConnectionService
in favor of null check and error logging.
- Modify rejectCallAndLog to skip calling reject on the incoming call if it
is already destroyed.  This is due to a call which was ended before the
call filtering completes.

Bug: 31236443
Change-Id: I0f93c65697a1b8e697a49118d642ecffe3dea130