Make PropertyTransformation public

This CL makes the PropertyTransformation interface public. This is the
first step to making it easier to apply custom transformations to
elements during transitions.

In follow-up CLs, I will:
 - add transformSize(), transformOffset() and transformAlpha() to the
   DSL.
 - change the semantics of this interface a bit so that the returned
   value is the value for the current frame instead of the value being
   interpolated with the target value and the transition progress.

Bug: 376438969
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: com.android.systemui.scene_container
Change-Id: Iedf20e47320686bf2886171b5154402b30aca4c9