Ensure only distinct light reveal alpha gets processed

There was a race condition on boot with the light reveal animation
that could inproperly leave the light reveal in a transparent state
when it should be opaque.

Fixes: 414817175
Test: reboot and check foldable
Flag: com.android.systemui.shared.ambient_aod
Change-Id: Ibb3fef34b5a275c93ee6ef12871eaebf2f57e9c8