[CS] 2/2: Define interface for NavigationBarController.

As we replace CentralSurfaces references with NavigationBarController
references instead, we need to allow other variants (specifically Arc)
to be able to provide an empty NavigationBarController instance (since
those variants do not have a CentralSurfaces instance.)

Bug: 277764509
Test: smoke test of nav bar
Change-Id: Ieb0f3f554bef7fb72523381285e0a373ccf13559