Let createSetStateAnimator return null when needed

- This is to fix a regression from http://ag/20968628. Instead of returning null and change the behavior, return null and do null checks on callers

Fix: 266109700
Test: Swipe AllApps from home on tablets, no flicker on taskbar, for gesture and 3-button
Change-Id: Ic1900ef396b00187ce9eced906c4628436481213