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Previously ambient mode will trigger lockscreen on screen dimm and then
rely on lock screen idle to show screen saver. This works in some cases,
but in suspend case the time between screen dim and cpu suspend is too
short. Screen saver was not able to enguage. The reason we have the set
up is because the screen saver was sharing the same window constainer as
the lock screen, it cannot show before lock screen is ready.

Now screen saver has its own window container, we changed the flow to
show ambient mode immediately when idle. This will prevent cpu suspend
because screen saver will take a wake lock if charging. Screen saver
will still lock the screen in the back if user reference indicated
lockscreen after wake.

Bug: b:169442907
Test: unitests and manual tests
Change-Id: I1a5df2c58c976bba492c709e0a61dd5a6314d084
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435334Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812397}