Use Transition.ReplaceOverlay.currentScene when getting placeholder size

This CL fixes the logic for the placeholder size of movable elements
during a ReplaceOverlay transition: we now use the size of the current
scene if the other scene never contained that element.

Bug: 373799480
Test: atest MovableElementTest
Flag: com.android.systemui.scene_container
Change-Id: Idc7501e80b8aa0f3cf6e7b6df424d2825cd5b737