[iOS] Move SettingsSwitchItem to Legacy

Preparation to create a new SettingsSwitchItem for TableView.

Bug: 894791
Change-Id: Icd88aa0a5af519172b0750782f3b029ac2cc7d29
Reviewed-on: https://chromium-review.googlesource.com/c/1296189
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601894}
25 files changed