[iOS] Add Safety Check notifications inactivity threshold overrides

This CL enables users to override the inactivity threshold that triggers
Safety Check notifications via Experimental Settings.

Change-Id: Ic18ea32261f83efb88c634be41dbe1b9eda0d505
Fixed: 364908913
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5836572
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Auto-Submit: Benjamin Williams <bwwilliams@google.com>
Commit-Queue: Benjamin Williams <bwwilliams@google.com>
Cr-Commit-Position: refs/heads/main@{#1352061}