[iOS] Move pref_names to the prefs/ folder

Bug: 1354614
Change-Id: Ife06d8c30941274ce0aa0221efcbcce34d15dbfa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3850516


Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1039144}