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Previously the animation was only initited from the ExpandArrowView
constructor which is now being called only once upon the AppView
construction.

Introduced a MaybeEnableHintingAnimation method to be called
every time the launcher state is changing.

Replaced PostTask with OneShotTimer to prevent multiple tasks
being scheduled in parallel. The timer is canceled whenever AppList
is closed.

Added a unit test.

Bug: 989181
Test: manual
Change-Id: I042ed4916497d2d70c900bdddcfce5a3b809342b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775069
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Auto-Submit: Vladislav Kaznacheev <kaznacheev@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691683}