Review Mozilla 1974290: PBM lock feature can't use experiment

🔍 Bugzilla Audit

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1974290

Description

That Bug ended up in a33112d6, which says "Refactor private browsing preferences".

I think it's worth having a deeper look.

Found as it conflicted with 9538f500 (I think privateBrowsingLockedEnabled was renamed to privateBrowsingLockedFeatureEnabled).

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