Fix the overscroll placement/composition logic

This CL fixes a broken logic in shouldPlaceElement() and
shouldDrawOrComposeSharedElement(): when we are overscrolling, we should
only place (or compose in the case of MovableElements) in the scene that
is overscrolling. Before this CL, we would both place (compose) in the
overscrolling scene *and* in the scene picked by the scene picker. This
means that when overscrolling a MovableElement could be composed twice,
which should never happen.

Bug: 290930950
Test: ElementTest
Flag: com.android.systemui.scene_container
Change-Id: I8f76d7b30b43434ec2e71af268fc5b51fe106ae2