Cancel previous IME window animation when switched

When IME1 is switched to IME2, IME1 doesnt finish animation and gets
stuck in exit animation and is never removed from screen.
An easy fix is to cancel the animation on IME1 so it can be removed when
IME2 is appearing.
Fix: 139488201
Bug: 111084606
Test: manually verified by switching IME using dialog
      and adb shell set.

Change-Id: Id34c9b325ebb68d4c14c2a12279c5a8410a8db0f