Make ViewModels consistent in terms of using ExclusiveActivateable

It's way more convinient to use flows in the View because of the
animations. This CL makes sure all Volume Dialog ViewModels follow the
same pattern of exposing properties as Flow.

This CL doesn't introduce new changes, but reorganizes the existing
code.

Flag: com.android.systemui.volume_redesign
Bug: 369994090
Test: manual on the foldable
Change-Id: I1818de4eee266edb70fc6216039a3db288230cb8