Login: Navigating back from the 2FA screen after selecting an account via Smart Lock loops back to the 2FA screen

Expected behavior

After selecting a WordPress account with 2FA enabled from the Smart Lock dialog on the Prologue, the user is redirected to the 2FA screen. Navigating back from that screen should redirect the user to either the Email/Password screen or the Prologue screen.

Actual behavior

The user gets stuck in a loop:

https://user-images.githubusercontent.com/830056/123467941-7f8bab00-d5c7-11eb-80dc-af6a0453bbd1.mp4

Steps to reproduce the behavior

1. Make sure you have a WordPress account with 2FA enabled saved on SmartLock.
2. On the Prologue screen, notice the SmartLock dialog.
3. Select the WordPress account that has 2FA enabled.
4. Notice a progress dialog on top of the Email/Password screen.
5. Notice you're redirected to the 2FA screen.
6. Press back.
7. Notice the progress dialog on top of the Email/Password screen briefly appears again.
8. Notice you're back on the 2FA screen.

Tested on Motorola One, Android 10, WPAndroid 17.6-rc-4

Thanks to @AliSoftware for the original report 🙇‍♂️

Start by reproducing the Smart Lock flow from the Prologue with an account that has 2FA enabled, then follow the Email/Password and 2FA screens while pressing back. Compare the observed navigation with the expected return to the Email/Password or Prologue screen. Done means pressing back from 2FA no longer loops back to 2FA.