Prevent outdated layers to update Element.SceneState.lastAlpha

This CL ensures that the graphic layer used to fade elements does not
update SceneState.lastAlpha once that layer is invalid/outdated. See
b/343138966 for details.

Bug: 343138966
Test: ElementTest
Flag: com.android.systemui.scene_container
Change-Id: Iad78755d706f1059441c5f8f188c42c4fbcaaed3
3 files changed