tests/DirectRenderingClusterSample/res/drawable/gradient_bottom.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
  <gradient
      android:startColor="#00000000"
      android:endColor="#FF000000"
      android:angle="270"
      android:dither="true"
      />
</shape>