Ensure that movable elements are composed even when not in a transition

This CL fixes a bug introduced with overlays: if a movable element is in
an overlay that is not part of any transition, that element would not be
composed at all anymore, even if the overlay is still shown.

Bug: 373799480
Test: atest MovableElementTest
Flag: com.android.systemui.scene_container
Change-Id: Ic875ea39f5351321c58d4c1de3fccda79d94de00