Autofill: Increase ratio of complete credential saves

[CDRussell]

Task/Issue URL: https://app.asana.com/0/72649045549333/1206048666874234/f

Description

Increases the ratio of complete credential saves by being able to capture a username-only form submission, and then re-attach it to a nearby password-only form submission. This is useful for scenarios like resetting passwords and multi-step logins.

Steps to test this PR

DRAFT: test sites:

• fill.dev / autofill.me
• deviantart.com

Logcat filter: message~:"Partial save" message~:"Pixel sent: m_autofill_logins_update_password_inline" message~:"Determined that username" message~:"Determined that username”

Simulating a multi-step login form

• Ensure no passwords currently saved (or none for autofill.me at least)
• Visit https://autofill.me/form/login-simple
• Enter username test
• Leave password blank
• Hit Login button
• Now clear the username
• Enter password (>=4 characters)
• Hit Login button (which is submitting only a password now)
• Save password when prompted, then go view it in the Password Management view (e.g., tap on View in snackbar)
• Verify it has username=test and password matches what you provided (i.e., the username was backfilled because the partial form submission’s username was later applied to the final form submission which didn’t have the username)

[CDRussell]

• Modify autofill update credential pixel to include backfilled parameter #5392
• Autofill: Increase ratio of complete credential saves #5386 👈 (View in Graphite)
• develop

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[CDRussell]

these are types specifically for Moshi / JSON parsing, used internally in this class. but we don’t necessarily want the same types returned out of this class

for example, the trigger is nullable when parsing from JSON, but when returning we also want that non-nullable and defaulting to UNKNOWN if it couldn’t be parsed from the JSON