Remove DepartingAnimationTarget from BackMotionEvent(2/2)

This would create another SurfaceControl object through IPC, which is
unnecessary because the animation should run through
BackAnimationRunner.

Flag: EXEMPT bugfix
Bug: 395035430
Test: execute back-to-home transition several times, verify no
predict_back animation leash left in offscreen layer.

Change-Id: I96176100fb28c24a635415799e0ed7a590ee8250
1 file changed