Don't clear the transient state before the end of hiding animation

Before this CL, the transient state would be cleared before the hiding
animation, so the semi-transperent background of system bar would be
gone suddenly. That makes the hiding animation inconsistent with the
showing animation.

This CL aborts the transient state when the client stops playing the
hiding animation. So the semi-transparent background will be there
during the animation.

Fix: 312137467
Flag: EXEMPT bugfix
Test: Open a YouTube video in fullscreen mode, swipe to show system
      bars, and wait for it to hide automatically. See if the semi-
      transparent background of the bars stays visible before the
      animation ends.
Change-Id: If3fa61005f7905bb541743e61ec20a6c8cf8a1f5