Debounce volume updates in the dialog

This change makes the coroutine to suspend until the volume changes, so
only the last volume change remains. This brings Volume Dialog behaviour
onpar with the Volume Panel.

Flag: com.android.systemui.volume_redesign
Fixes: 419818348
Test: manual on foldable. Rapidly adjust volume and observe that there
are no movements after the grace period

Change-Id: Ibc668eb2233536e5b712320dcf34e8aac98a1904