[Android] You are not logged in if two "Sign In" windows are opened

[Softvision-MariusComan]

[Affected Platforms]:

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

[Prerequisites]:

• Have the latest version of the "Firefox Notes" application (app-release-signed) custom built on 8/1/2018 installed.
• You are not logged into "Firefox Notes".

[Steps to reproduce]:

1. Open the Notes application and tap the "Sign In" button.
2. Tap the system's "Back" button and quickly tap the "Sign In" button again.
3. Log in using a valid account.
4. Observe the behavior.

[Expected result]:

• You are successfully logged in.

[Actual result]:

• You are not logged in and you are returned to the first opened email screen.

[Notes]:

• This issue is still reproducible after the app is reopened.
• Attached a screen recording of the issue:
  

[Softvision-RemusDranca]

I have verified this issue using the latest "Firefox Notes" application (app-release-signed) custom built on 8/21/2018 (https://circleci.com/gh/mozilla/notes/2938) and the issue is still reproducible.
Tested on Android 8.0.0 (Google Pixel); Android 8.0.0 (Samsung S8) and Android 6.0 (Huawei P8 Lite).
Considering this I will reopen the issue.