Allow WindowState to assign layer during transition.

But don't build the layer info into finish transaction.
So when an Activity contains multiple WindowState, the layers can be
update according to the hierarchy order. Keeps the layers as zero can
confuse surface flinger. Also this shouldn't affect transition since
window state won't participat transition directly.

Bug: 270906945
Bug: 275459505
Test: manual, launch app and monitor the z order of windows can change
during transition animation.

Change-Id: I10daa1c4d9403dc29e5c740670d43a7590f647c0