title: layout_gravity doesn't work when parent layout is ConstraintLayout
body: <p>I know that layout_gravity = "center" would center the current View or Layout in it's parent layout.</p>
  
  <p>In the code example I have given, Both the first TextView and the LinearLayout cannot have layout_gravity as a property when their parent is ConstraintLayout.</p>
  
  <p>However, the second TextView and the second LinearLayout can have layout_gravity property when their parent is a LinearLayout.</p>
  
  <p>Why is that so?</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" android:layout_width="match_parent"
      android:layout_height="match_parent"&gt;
  
      &lt;TextView
          android:layout_width="wrap_content"
          android:layout_height="wrap_content" /&gt;
      &lt;LinearLayout
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:orientation="vertical"&gt;
  
          &lt;TextView
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:layout_gravity="center"
              android:text="heey"/&gt;
  
          &lt;LinearLayout
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:layout_gravity="center"&gt;&lt;/LinearLayout&gt;
  
      &lt;/LinearLayout&gt;
  
  &lt;/android.support.constraint.ConstraintLayout&gt;
  </code></pre>