Update NavBarController's CommandQueue callbacks to be separate.

Moves the CommandQueue.Callback methods to be in a separate object to
make it clear what interface the methods are overriding. Child CL will
add additional override methods.

Bug: 277764509
Test: smoke test of gesture nav & 3 button nav working
Test: atest NavigationBarControllerTest
Change-Id: I7fb46a09be9c5a719679887948aa6c60aeddfe31