Fix preserving currently visible slider

Flag: com.android.systemui.volume_redesign
Bug: 369994956
Test: atest VolumeDialogSlidersInteractorTest
Test: manual on the tablet. Join the call to force the dialog to show
media and call sliders at the same time. Adjust any volume.

Change-Id: Ib6445bd8e02abab9974d1478a8e38c2342faaf58