Don't create a derived State for Element.alpha

This CL changes the logic in Modifier.element() so that we don't create
a derived State object for each element. This paves the way towards
using the Modifier Node API for Modifier.element().

Bug: 291566282
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: NA
Change-Id: Ia778559659a44e4cb7785882cf56e9bb67e2d803