Replace connectToCell -> NetworkCallbackRule

Developers often forget to unregister requests in tests, and cleanup
code also makes tests more verbose.

Add a rule that unregisters callbacks automatically when after each test
method.

Replace current usage of connectToCell with this rule, as well as
network callback usage in CaptivePortalTest (which is one example that
did not unregister its callback).

Test: atest
Change-Id: Ie3b9c5cf529004b7b3af04bc13a7e69acfbbaa18
8 files changed