title: Fade in, fade out not working in MotionLayout
body: <p>I'm trying to make an effect of fade in, fade out using alpha and motionLayout, but it seems that it's not working.</p>
  
  <p>This is the imageView that I want to fade.</p>
  
  <pre><code>    &lt;ImageView
      android:id="@+id/tijeras"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_marginStart="8dp"
      android:layout_marginTop="16dp"
      android:layout_marginEnd="8dp"
      android:contentDescription="@string/tijeras"
      android:src="@drawable/ic_tijeras" /&gt;
      &lt;/android.support.constraint.motion.MotionLayout&gt;
  </code></pre>
  
  <p>And this is the motion file</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;MotionScene xmlns:app="http://schemas.android.com/apk/res-auto"
      xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;Transition
          app:duration="2000"
          app:constraintSetStart="@+id/start"
          app:constraintSetEnd="@+id/end"&gt;
          &lt;KeyFrameSet&gt;
              &lt;KeyAttribute
                  app:framePosition="0"
                  app:motionTarget="@id/tijeras"
                  android:alpha="1.0"/&gt;
              &lt;KeyAttribute
                  app:framePosition="50"
                  app:motionTarget="@id/tijeras"
                  android:alpha="0.0"/&gt;
          &lt;/KeyFrameSet&gt;
      &lt;/Transition&gt;
      &lt;ConstraintSet android:id="@+id/start"&gt;
          &lt;Constraint android:id="@+id/tijeras"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:alpha="1.0"
              app:layout_constraintEnd_toEndOf="parent"
              app:layout_constraintStart_toStartOf="parent"
              app:layout_constraintTop_toBottomOf="@+id/logo"/&gt;
      &lt;/ConstraintSet&gt;
      &lt;ConstraintSet android:id="@+id/end"&gt;
          &lt;Constraint android:id="@+id/tijeras"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:alpha="1.0"
              app:layout_constraintEnd_toEndOf="parent"
              app:layout_constraintStart_toStartOf="parent"
              app:layout_constraintTop_toBottomOf="@+id/logo"/&gt;
      &lt;/ConstraintSet&gt;
  &lt;/MotionScene&gt;
  </code></pre>
  
  <p>I can see the image, but it's not doing the alpha in and out. Any idea? Do I need to trigger it for starting?</p>