Add helper methods to CommunalInteractor

These are helpful for external transitions to depend on in order to
know when the hub is fully showing and not in transition.

Bug: 315205216
Test: atest CommunalInteractorTest
Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT
Change-Id: I3f746a2c6339c924113ef0f8fbd85adfe0fd5f08