Add callback for enabling/disabling window magnification

To support switching magnification mode, we add callback to know
when the animation is finished. We only invoke the newest callback
when the animation is end successfully or the spec didn't change

Bug: 161669184
Test: atest com.android.systemui.accessibility
      atest com.android.server.accessibility.magnification
      Enable window magnification to see if it works normally
Change-Id: I34ce39d133a000964d1089946b7bcf510a618b40