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changes in this cl:
- For clamshell devices, which we should not turn screen off by tapping the
  power button. Starts the menu animation immediately as press the power button.

- Make the initial opacity in the menu animation not fixed, use the value in
  previous animation.

- Still delay the power menu animation if we should turn screen off by tapping
  the power button. And differeniate the timer fire time according to the initial
  scrern state.

Bug: 799524
Change-Id: Icad6c4e66be2ec41978456862c337ff494bfac00
Reviewed-on: https://chromium-review.googlesource.com/953374Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Qiang Xu <warx@google.com>
Commit-Queue: min c <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542227}