Review Mozilla 1910669: Implement StorageArea.getKeys() in extension storage API

🔍 Bugzilla Audit

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1910669

This refactor conflicted with Mullvad Browser's patch MB 47: uBlock Origin customization while rebasing 143.

We should verify that the conflict resolution was properly done.

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