[Android] "Confirm your account" page is wrongly redisplayed after tapping the "Back" button twice if the account is not confirmed

[Softvision-AndreeaCupsa]

[Affected Platforms]:

• Android 8.0.0 (Google Pixel, Samsung S8)
• Android 6.0 ( Huawei P8 Lite)

[Prerequisites]:

• Have the latest version of the "Firefox Notes" application (app-release-signed) custom built on 08/31/2018.

[Steps to reproduce]:

1. Open the Notes application.
2. Enter a valid email and tap the "Continue" button.
3. Enter a valid password and age.
4. Tap the "Create account" button.
5. Tap the "Back" button twice and observe the behavior.

[Expected result]:

• The "Enter your email" page is displayed.

[Actual results]:

• The "Confirm your account" page is displayed.

[Notes]:

• Tapping the "Back" button won't dismiss the "Confirm your account" page after it is wrongly redisplayed.
• Double tapping the "Back" will display the "Sign in" page and the following message: "Something went wrong. Error: Failed to authenticate" after the "Confirm your account" page is wrongly redisplayed.
• Attached a screen recording of the issue:

[vladikoff]

Thanks! filed https://github.com/mozilla/fxa-content-server/issues/6408