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- Remove Attempt Enter/Leave TabletMode() which was scattered
  and used all over the place.
- Make a clear distinction between a device physical tablet
  state (which is affected by device events such as lid angle
  changes, tablet EC switches, ... etc.) and a UI tablet state
  (which is whether the UI is currently in tablet or clamshell
   modes).
- Gather all the inconsistent and scattered logic that affects
  the device and UI states into consolidated, and clear functions
  that get invoked whenever the state needs to be updated.

This CL is a prerequisite work for the auto-rotation work
coming up soon.

BUG=925087
TEST=No behavior change, all tests must pass.

Change-Id: If4c2777f434a09f3c70b32bb60367cfb118102b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1851286
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704975}