STL ElementStateScope exposes approachSize()

This value is calculated in the approachMeasure phase, before the
children's measure is calculated, so the value can be used by the
children to calculate their size or as input to a MotionValue.

Test: atest ElementTest
Bug: 404526497
Flag: com.android.systemui.scene_container
Change-Id: I79ba8fb7a1de091d51b61f3e9b7e05dfa1327873