polish(settings): Add callback so calling code knows when MfaGuard is dismissed.

[dschom]

Because

• This will be needed by FXA-12230, but is probably also generally useful.

This pull request

• Adds an onDismiss callback option to the MfaGuard

Issue that this pull request solves

Closes: (issue number)

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• 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).

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