fix(nimbus): Use uid if available for nimbus user id

[vbudhram]

Because

• We want to use the FxA uid if the user is signed in

This pull request

• Uses the FxA uid for local account

Issue that this pull request solves

Closes: https://mozilla-hub.atlassian.net/browse/FXA-12697

Checklist

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Other information (Optional)

Any other information that is important to this pull request.

[LZoog]

nit: this isn't necessarily a legacy local storage account, could call it something else here?

Though, potentially an actual issue: this won't update if a user hits Accounts and then signs in or signs up, React won't be reactive to the local storage update to be able to rerun this useEffect based on that dep. This sort of relates to that other follow up issue I filed... I assume we want to address this, and not use the Nimbus uniqueUserId when that happens? LMK if so and if you'll address that elsewhere and I'll r+ this one.

[vbudhram]

@LZoog Yea, this one will be addressed in an upcoming PR. It turned out to be pretty hairy and wanted to keep the PRs separate.

[LZoog]

Makes sense, will r+ this then 👍