Unicorn: Chrome activity reporting in ChromeOS

Bring back the deleted code https://crrev.com/c/3707983.

The feature is guarded behind the feature flag with disabled by default.

Remove `kActiveAllowlisted` in ChromeAppActivityState since it's similar
to `kActive` and there isn't a need to differentiate them.

Verified locally in ga/test that the event was successfully sent,
screenshot attached in comment#16 of the linked ticket.

bug: 413082634
Change-Id: Id350b111810cc8097c8099eb887acc6366289da0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6731268
Reviewed-by: Longbo Wei <longbowei@google.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Wenyu Zhang <zhangwenyu@google.com>
Cr-Commit-Position: refs/heads/main@{#1506149}