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This CL handles the system suspend/resume events accordingly when
ambient is on.

Misc:
Minor refactoring including: rename functions and use
|ScopedObserver| instead.

Bug: b/157252087
Test: covered by unittests.
Change-Id: I3a1244e2aa5e47fd83082a7ada40e5b2fb5aabd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2270676
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785983}