DeskClock: Properly align settings

* The alarm volume setting doesn't look as it should
* Copied "preference_volume_slider" from Settings (used in Settings->
  Sound) and stripped by stuff we don't need (suppression_text,
  widget_frame)
* Looks like without providing an initial icon, the layout would not
  inflate the space for it properly, so provide the default one

Change-Id: I31d103cd0b0a6f096d376af2026702a0a4754671