The app bar displays a back arrow and exits the setup
flow when the back arrow is pressed. However,
exiting the flow causes the user to lose all progress. 
Therefore, the user is prompted to confirm whether they
want to exit the setup flow.

Prompt the user to confirm exiting the setup flow,
and ensure that the prompt appears when the user
presses the hardware back button on Android.

When the user taps the back arrow in the app bar,
or presses the back button on Android,
an alert dialog pops up to confirm that the
user wants to leave the setup flow.
If the user presses Leave, then the setup flow pops itself 
from the top-level navigation stack.
If the user presses Stay, then the action is ignored.

You might notice that the Navigator.pop()
is invoked by both the Leave and 
Stay buttons. To be clear,
this pop() action pops the alert dialog off 
the navigation stack, not the setup flow.