Update AmbientVolumePreference to match expressive design

AmbientVolumePreference is a preference group that always expanded to
show at least one child preference. Make it expandable to let the
adapter render the rounded background correctly.

Flag: EXEMPT bugfix
Bug: 409715235
Test: manually check the UI
Change-Id: I6b682acb055aa91c449147652d612ee4020c98d9