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Currently ambient mode will exit on key press. However, the key event is
not consumed and will enter into the login pod.
To be consistent with other platforms, the key press is consumed in
ambient mode by making the ambient controller a pretarget event handler
and stopping propagation of the key event.

Bug: b/173642446
Test: new unittests
Change-Id: Iad04b07a0e6124773326c1d52d4a48a919ab87fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626911
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843970}