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Record the number of users who enabled/disabled Assistant service on their
devices, the sum would be the number of all eligible users. Note that
we simply report this metrics with every UMA upload and should only depend
on unique users count to take advantage of the de-dupe logic implemented
on the server-side. Ignore potential reboot or crash for now.

Test: local compile and manually test.
Bug: b:117518834
Change-Id: I5cee656db986f60104999502b4feebb8d072c79f
Reviewed-on: https://chromium-review.googlesource.com/c/1292111
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610229}