Implement correct semantics for Android back button navigation through the app
In #45 (closed) we integrated the Android back button with navigation through the app. Specifically, we made the back button navigate backwards through location history - all the way to the first route, the "main" screen with the "I'm Ready" button.
We probably don't want the back button to be able to navigate all the way back to this "main" screen. We may also want multiple uses of the back button to be usable to exit the app in some contexts.
First step, specify exactly what behavior we want.