Reodering some drawables

  > quantum_panel_shape is xml rectangle with no margin
  > quantum_panel_bitmap is 9-patch drawable with inbuild shadow and margin
  > quantum_panel uses xml-shape for lollipop and 9-patch image for lower
    devices. It includes margin. For shadow, use elevation on lollipop
  > All drawables have _dark variant
  > Using #f5f5f5 for panel background, instead of #ffffff

Change-Id: Ia7c6cea6d514ee3774fcf5fef6139650bdd53208