Merge Content modifiers into Content{Element,Node}

This CL refactors the modifier chain used in Content into a ContentNode,
using the Modifier Node API. This is a pure refactoring that will allow
ag/32131525 to do something in onDetach() without having to add a new
DisposableEffect.

Bug: 400688335
Test: Existing tests
Flag: com.android.systemui.scene_container
Change-Id: Iccac180e3f3f8daa887cdba45bb3227510819698