title: Imageview constrained to bottom of toolbar filling toolbar
body: <p>Im using a constraint layout with an imageview inside it and I constain the top of the imageview to the bottom of the toolbar, I want the image to fill the rest of the screen though depending on screen size, so I set the imageviews hieght and width to math-parent and also scaleType=fitXY. 
  The problem I have is that even though it is constrained to the bottom of the toolbar it fills the toolbar as well.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>&lt;ImageView
      android:id="@+id/background"
      android:scaleType="fitXY"
      android:src="@drawable/rain_drops"
      app:layout_constraintTop_toBottomOf="@id/toolbar"
      android:layout_width="match_parent"
      android:layout_height="match_parent"/&gt;
  &lt;!--android:alpha="0.5"/&gt;--&gt;
  
  
  
  
  
  &lt;android.support.v7.widget.Toolbar
      android:id="@+id/toolbar"
      android:layout_width="match_parent"
      android:layout_height="?attr/actionBarSize"
      android:layout_alignParentTop="true"
      android:elevation="6dp"
      android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
      app:popupTheme="@style/ThemeOverlay.AppCompat.Light"/&gt;
  
  &lt;android.support.design.widget.TabLayout
      android:id="@+id/tab_layout"
      android:layout_width="match_parent"
      android:layout_height="?attr/actionBarSize"
      android:layout_below="@+id/toolbar"
      android:elevation="6dp"
      android:theme="@style/ThemeOverlay.AppCompat.Light"
      app:tabSelectedTextColor="@android:color/white"
      app:tabTextColor="@android:color/darker_gray"/&gt;
  </code></pre>
  
  <p>Thanks</p>