title: Show Toolbar shadow only on scroll in CoordinatorLayout
body: <p>I would like to know how I can use a Toolbar / CollapsingToolbarLayout in a CoordinatorLayout to show a shadow only when content is scrolled under it and not otherwise (i.e. after the fragment is created it should not show a shadow until the user starts scrolling down a list). Preferably with just XML</p>
  
  <p>(I don't think I need a AppBarLayout here, since the size of the toolbar is fixed and does not need the resizing or parallax effects etc..)</p>
  
  <p>Any help with this is much appreciated</p>