Make ConstraintLayout the root of the VolumeDialog

This achieves two purposes:
1) Allows us to align the dialog based on the slider position instead of
   simply centering it;
2) MotionLayout transition for half-folded state to animate dialog
   position when folding;

Flag: com.android.systemui.volume_redesign
Bug: 369994090
Test: manual on the foldable
Change-Id: I54868663984544fb3809e9e2b8c6fe7e16ee058d