Investigate why BackPressedCallback is disabled

[jhugman]

In #804 we found that the BackPressedCallback listener is not being enabled. We're not sure why.

Current theory is that it was disabled when the AppRoutePresenter and AutofillRoutePresenter was being merged.

Without it, the back stack history (as maintained by the RouteStore is likely getting out of sync with the actual back stack). Recall that the RouteStore maintains the current route so whenever the app is backgrounded, it needs to detect or fakeout what the actual back stack is doing.

Outcomes of this:

• re-enable the back button pressed callback for both route presenters.
• better understanding of the autofill route presenter — especially around testing. That no tests are failing either way is a problem.
• or deleting the callback all together.