Fix getting submitted username for Credential Banner

[varjolintu]

Sites with two phase login (a separate username and password field pages), when showing the Credential Banner, the username has no longer a valid input field inside the form. The value retrieved for the banner is empty, or incorrect value from another text input.

Instead, rely on the page_get_submitted request that has the previously entered username in the background script's memory. If the form username differs from the submitted one, use the previously submitted username. Pages where the login form that has both fields, the page_get_submitted returns an empty object, and the username value from the form is used normally.

• Fixes After login always new entry with username 1 pop up on sparkasse website #2981

Testing strategy

Use site https://www.blsk.de/de/home.html or Microsoft login page to test the fix. Without the fix, username for the first page is always "1". With Microsoft's login page, the username is empty.

Type of change

• ✅ Bug fix (non-breaking change that fixes an issue)