Revert "STL ElementStateScope exposes lastSize()"

This reverts commit 1faa0111.

Reason for revert: Not needed anymore after ag/34941048

Remove `ElementKey.lastSize()` from public API, this API was initially
introduced with ag/32945988.

The function was backed by a `mutableStateOf` to provide an element's
current size during transitions. However, we cannot make assumptions
about when this state might be read by consumers. If it is read after
the layout phase, it can trigger an additional composition/layout pass,
leading to a series of unexpected bugs and performance issues (see
b/431146033).

To prevent this, `Element.State.lastSize` is no longer a
`mutableStateOf`. The public API and its corresponding tests are also
removed.

Test: Manually tested on the demo app
Bug: 419520966
Bug: 404526497
Flag: com.android.systemui.scene_container
Change-Id: I8138ddba0234ea43d7433fb3b7c4169ce315d5b1