[Reauth] Make the preference available on ios

This preference will be default true, controlling the availability of
mandatory reauth flow

Bug: 1427216
Change-Id: Icdc13a8c3da353c149a2580e2e37e9d99299e2cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4781838
Commit-Queue: Siyu An <siyua@chromium.org>
Reviewed-by: Olivia Saul <jsaul@google.com>
Reviewed-by: Christoph Schwering <schwering@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1186714}