Don't elevate during overscroll

This CL disables element elevation when a transition is overscrolling.
This is necessary because some overscroll effects are applied in the
drawing phase, so elements have to be drawn in their original scene
durign overscroll.

Bug: 409305420
Test: Manual, see b/409305420#comment1
Flag: com.android.systemui.scene_container
Change-Id: I731ddfffc262465718220e8ac7489972d1cb9c48