Rename Element.TargetValues to Element.SceneState

This CL is a pure renaming of classes and fields of the Element class
to avoid confusion betwen shared/animated values and the captured state
of an Element in a given scene.

Bug: 291071158
Test: AnimatedSharedAsStateTest
Flag: N/A
Change-Id: Ic456cd75f572c43943ab9cfe8469eb5944e2f8a5