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The floating keyboard should not cause the shelf to disappear. Although
the regular full-width keyboard should.

This CL does 3 things:
- Uses the keyboard's occluding bounds rather than visual bounds. This
  describes the area that is not accessible due to the keyboard
  covering it. This will be empty bounds in the case of a floating
  keyboard.
- Uses keyboard availability event rather than checking if bounds are
  empty to determine if the keyboard is available.
- Uses the is_locked attribute on the state descriptor object rather
  than checking the GetInstance()'s value directly. There are too many
  places where code is querying the static instance directly for the
  keyboard's current lock state and so we'd like to...lock those down.

Bug: 786290
Change-Id: I8aba795b7228a14d00a8aaf218d4f56fb1006091
Reviewed-on: https://chromium-review.googlesource.com/816483Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Blake O'Hare <blakeo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523998}