title: app:layout_marginBottom is not working well with android constraint layout
body: <p>Is there any reason why the following layout_marginBottom is not working?
  However, if I use layout_marginTop on the second view it does work well</p>
  
  <pre><code>&lt;android.support.constraint.ConstraintLayout
      xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:background="#ade4ad"&gt;
      &lt;TextView
          android:id="@+id/first"
          android:layout_width="90dp"
          android:layout_height="40dp"
          app:layout_marginBottom="10dp"
          android:background="#000"/&gt;
      &lt;TextView
          android:id="@+id/second"
          android:layout_width="90dp"
          android:layout_height="40dp"
          android:background="#fff"
          app:layout_constraintTop_toBottomOf="@+id/first"/&gt;
  &lt;/android.support.constraint.ConstraintLayout&gt;
  </code></pre>