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When assistant is started and triggered hiding animation, during a very
short time window if the user lifted their finger, we would try to play
end animation.  We should avoid doing that.

BUG=b:67496461
TEST=build and flash locally, see animation playing normally

Change-Id: Idf52f692edd9d05b16ad48687384d291b208a42e
Reviewed-on: https://chromium-review.googlesource.com/748268Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513208}