[Networking] Remove OperatorNameViewController as a SignalCallback

This class was only listening to `SignalCallback` to check for airplane
mode changes.

This CL swaps out the signal callback for a direct listener on
AirplaneModeInteractor. It also changes the view controller to be more
testable by injecting the SubscriptionManagerProxy. And now we have
tests!

Test: OperatorNameViewControllerTest
Bug: 291321279
Flag: NONE
Change-Id: I426539c2e04b87190ed256e64e7d881d86ee2bd7