AccuracyTips: Add finch parameter for site engagement

Depending on experiment results we might want to try higher or lower
engagement values, so the limit should be configurable.

Bug: 1214777
Change-Id: I01cb5b6b6872fc21657fc30eb77cef0a8caf19ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3086447


Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Olesia Marukhno <olesiamarukhno@google.com>
Cr-Commit-Position: refs/heads/master@{#911209}