Fork volume_dialog.xml for the BC25 update

This change renames the existing volume_dialog.xml to
volume_dialog_legacy.xml and adds a new volume_dialog.xml for the BC25
dialog version.

Flag: com.android.systemui.volume_redesign
Test: passes presubmits
Bug: 369994090
Change-Id: Ic18cc06f49eae7ff3e6d93ad200fbf96fb83b8f8