title: Center fixed-size ImageView horizontally in parent ConstraintLayout
body: <p>I have a rather simple layout: A <code>ConstraintLayout</code> filling the entire screen, with one big <code>CardView</code> that, at the top, has an <code>ImageView</code> that is half on the <code>CardView</code> and half above it (actually not half, but you get the point).</p>
  
  <p>However, there are two issues: the <code>ImageView</code> sticks to the left of the parent and I do not know how to make it horizontally centered - all I can find on Google are chains for multiple elements, but I just want one centered.</p>
  
  <p>The layout editor works extremely bad for me. My first complaint is that I seem to be unable to create constraints to the parent. Then I tried centering the <code>ImageView</code> horizontally in parent by pressing the button that says "Center in parent". This increases the <code>CardView</code>'s width, no effect at all on the <code>ImageView</code>.</p>
  
  <p>While I am at it, how could I z-order the <code>ImageView</code> on top of the <code>CardView</code>? </p>
  
  <p>Here the layout:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &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="@color/colorPrimaryDark"&gt;
  &lt;android.support.v7.widget.CardView
      android:layout_width="0dp"
      android:layout_height="0dp"
      android:layout_marginBottom="32dp"
      android:layout_marginEnd="32dp"
      android:layout_marginStart="32dp"
      android:layout_marginTop="160dp"
      app:cardCornerRadius="12dp"
      app:cardElevation="32dp"
      app:layout_constraintBottom_toBottomOf="parent"
      app:layout_constraintEnd_toEndOf="parent"
      app:layout_constraintLeft_toLeftOf="parent"
      app:layout_constraintTop_toTopOf="parent"&gt;
  
  &lt;/android.support.v7.widget.CardView&gt;
  
  &lt;ImageView
      android:layout_width="200dp"
      android:layout_height="200dp"
      android:layout_marginTop="40dp"
      android:scaleType="centerCrop"
      android:src="@drawable/re_zero"
  
      app:layout_constraintHorizontal_bias="0.5"
      app:layout_constraintStart_toStartOf="parent"
      app:layout_constraintTop_toTopOf="parent" /&gt;
  </code></pre>
  
  <p></p>
  
  <p>Thanks!</p>