Reapply "Don't place the modal clickable for invisible overlays"

This reverts commit 956bc2a0.

This CL is a reland of ag/34828325, which was broken given that it was
not applying the `ContentElement` modifier on the scenes and only on the
overlays, and caused b/435361598. This regression is now covered by the
tests added/improved in ag/34917382 and ag/34918781, which pass with
this CL.

I also double checked that b/435361598 was not happening with this
change.

Bug: 433909610
Bug: 435361598
Test: atest SceneTransitionLayoutTest
Flag: com.android.systemui.scene_container
Change-Id: Ic0860d269a77e86c4638890fd48f388f0ce23a6c