[CS] Unregister callback as part of onUnavailable dispatch

The onUnavailable semantics promise that it is equivalent to calling
the unregister callback method. But - it doesn't unregister the callback
allowing it to be reused. Fixed.

Additionally, modified the unregisterNetworkCallback method to not fail
on duplicate unregistration (since a callback could now self
unregister). Instead simply print a log.

Bug: 130651445
Test: atest ConnectivityServiceTest
Change-Id: I4c54b003a733eb0b1e4fd8674ed13081b1bef8e3