[Android][Enhancement] Add a loading spinner to the "Continue" button from the "Account verified" page

[Softvision-MariusComan]

[Notes]:

• Considering the fact that there is a delay between the tap on the "Continue" button and the actual login, we consider that adding a loading spinner is necessary in order to understand that the app is working in background in that period of time.

[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 7/27/2018 installed.
• You are not logged into "Firefox Notes".
• Have an email that is not linked to a "Firefox Account".

[Steps to reproduce]:

1. Open the Notes application and tap the "Sign In" button.
2. Enter the email from prerequisites and tap the "Continue" button.
3. Complete the fields and tap the "Create account" button.
4. Confirm your account.
5. Switch back to the "Notes" app and tap the "Continue" button.
6. Observe the behavior.

[Expected result]:

• A loading spinner is displayed on the "Continue" button and you are logged in.

[Actual result]:

• The button remains active and can be tapped multiple times.

[Additional Notes]:

• Attached a screen recording of the issue: