Always store the target state in a scene

This CL ensures that we always store the target state of an element in a
scene, even if it is not placed in that scene (for instance because we
are overscrolling on the other transition scene).

Bug: 27298915
Test: ElementTest
Flag: com.android.systemui.scene_container
Change-Id: I4ae91fa3f7c8ad33375682eed037910a6bdfed91