[Bedtime Hours] Prevent tampering with system time

In order to prevent students from getting out of the restricted mode,
prevent tampering with the system time.

Remember the highest seen system time and write some logic around it
(since the system time can be changed by removing the CMOS battery).

Bug: b:345186543
Change-Id: I9d2dbe4769d5340b1421f5a2cb8663c8d083a34e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5971575


Reviewed-by: Sergey Poromov <poromov@chromium.org>
Commit-Queue: Ivan Šandrk <isandrk@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378928}