Move the Element placement logic into ElementNode

This CL is a pure move of the place() logic into ElementNode. This will
be useful for ag/27685078, which will make the placeWithLayer {} block
access the list of current transitions (ElementNode.currentTransitions).

Bug: 290930950
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: com.android.systemui.scene_container
Change-Id: I5b882188b8f7073369d635795227400b8aeca2ee