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Split out some helper classes into their own files in the process:
HotseatStateWatcher, OverviewAnimationWaiter,
ShelfLayoutManagerTestBase.

This is a pure (albeit large) refactoring and does not modify any of the
actual test semantics.

Change-Id: Ica79e3dcead839daa83022de329b5aca8f636c40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015672
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734615}