Always draw new Expandables in a graphics layer

This CL makes all Expandables draw their content in a graphics layer,
ensuring that we don't make assumptions about drawing order between the
Expandable window and the dialog window. See b/397904604#comment6 for
details.

Bug: 397904604
Test: Manual
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Change-Id: Ia753d96a264dde46caec9e389c223889b8c020e5