Fix Volume Dialog vertical margins for display large scale

The idea is that the Volume Dialog should actually stay in the middle of
the physical screen. This change compensates slider margins with the
insets to make this happen. This results in a smaller total margins
between the dialog and the edges of the screen leaving more room for it
when the display is on its largest scale.

Flag: com.android.systemui.volume_redesign
Fixes: 412572353
Fixes: 412566986
Test: atest VolumeDialogScreenshotTest
Test: manual of foldable with max display scale. Observe the Volume
Dialog appearance

Change-Id: Ic4c31a04290599db79d935bf42973ee7552b6580