Make SceneTransitionLayoutTest#alwaysCompose(Overlay) more robust

This CL makes the STLTest#alwaysCompose(Overlay) tests more robust so
that they don't rely on the additional logic in Modifier.element() to
correctly test the alwaysCompose behaviors of scenes and overlays.

This improvement would have caught the regression at b/435361598, caused
by ag/34879256.

Bug: 435361598
Test: atest SceneTransitionLayoutTest
Flag: TEST_ONLY
Change-Id: I9c7de36129f214da8760a9474fe2b95ad85ecb31