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This adds ability to enforce throttling mode and adds obsever to receive
throttling change updates.
DD: go/arc_power_control

TEST=Unit test + locally in context of prototype.
BUG=b:170982650

Change-Id: I1fe3633ff5792355917c890280155a8b5d73892f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2477454
Commit-Queue: Yury Khmel <khmel@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818748}