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Changes in this CL:
- Remove have_seen_accelerometer_data_ variable. As this variable is not
  set if the device doesn't have kAshEnableTabletMode flag.
- Only observe the display/lid/input device events if the ui mode is
  allowed to be changed.

No functionality is changed in this CL.

Bug: 887042
Change-Id: Ic42d4cc0c25667e7aa67efcf26f65032082db126
Reviewed-on: https://chromium-review.googlesource.com/1244289Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594407}