STL ElementStateScope exposes lastSize()

This method returns an observable *last known size* of an Element.

## Important note
Usually updated **after** the measurement pass and after processing
children.

However, if the target size is known **in advance** (like during
transitions involving transformations or shared elements), the update
happens **before** measurement pass. This earlier update allows children
to potentially use this predetermined size during their own measurement.

This change has been discussed in ag/32426737

Test: atest ElementTest
Bug: 404526497
Flag: com.android.systemui.scene_container
Change-Id: I0a5821d557f38300a75a83d4013f385893b15c85