[iOS] Allow 21 days for data collection before showing promo

Trigger criteria experiment is meant to show the default browser promo
by skipping eligibility conditions and record  stats related to trigger
criteria at the time of the promo display.
For this we need to allow some time for collecting stats before showing
the promo.

In this CL, add a new FET event that will make users in this experiment
eligible for default browser promo if the experiment has been active for
them at least 21 days.

Bug: crbug.com/326260960
Change-Id: I3257f79de665d23171e602ccf4c2ab51a40e7c78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5608294


Commit-Queue: Gayane Petrosyan <gayane@google.com>
Reviewed-by: Ginny Huang <ginnyhuang@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/main@{#1312810}