Fix regression in Onion Services authentication prompt focus

The patch for #41856 (closed) was removed at some point (probably in a rebase), so we've regressed it.

STR are the same:

1. Open http://pierovlcy7baatz7xcaccbf2kanct3hvkhcgedz4cbrmcg2ybmjalxad.onion/ and wait for the authorization prompt
2. Move the focus to a different window
3. Click back onto the prompt (which should be visible because is rendered on top of all the other windows)
4. Try to type into the "Enter your privae key for this onion service" text field

The old patch needs just minor adjustments for conflict resolution.