Review Mozilla 1807553: [meta] Biometric authentication for private mode

🔍 Bugzilla Audit

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1807553

Description

Adds an option to the Android browser to lock your private tabs behind some PIN/biometric entry. I'm guessing for when you switch apps and return to the browser, but I'm not sure.

Opening this as mostly a UX question, for how well this integrates with our default private browsing experience.

/cc @donuts @felicia @dan @clairehurst

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