[ECUK/EU] First biometric auth after log out/in sees unnecessary soft prompt #86284
Description
Problem
The first time a user uses MFA after they log out and back in, they see the soft prompt instead of moving right to authentication. The problem was revealed by #86059, though it wasn't caused by it.
Follow-up to #86059
Why is this is a problem?
It's not intentional and causes unnecessary friction.
Solution
The onyx key deviceBiometrics stores whether or not the user has seen the soft prompt. We intentionally store it in onyx so that a user who deletes and reinstalls the app on iOS will see the soft prompt. This is necessary because in that situation they will see the system prompt, and we always want them to see the soft prompt first. However it had the unintended side-effect of carrying the behavior over into log out/in. Storing deviceBiometrics in onyx will fix that, so we won't show the soft prompt unnecessarily on log out/in, only if the app is fully deleted or the value manually deleted from onyx in the browser.
In order to prevent this value's storage from affecting other users, we'll have to turn it into a collection and incorporate the accountID, like we do for stored passkeys.