Only unbind if bind() returned success

ServiceConnection may become disconnected asynchonously for various
reasons so calling unbind() on a disconnected SC is actually handled by
the framework. What we don't want to do is call unbind() on a service
which we never successfully called bindService() on, so in this CL we
will track the client-side binding instead of the connection status
through callbacks. This is less racy and handles the root cause of the
exception.

Bug: 145661922
Test: testUnbindWhenNotBound
Change-Id: Ia9bd746440faa047523a750afcae84c347de86d6