You CAN do it using only xml shapes - just use layer-list AND negative padding like this:
<layer-list>
<item>
<shape>
<solid android:color="#ffffff" />
<padding android:top="20dp" />
</shape>
</item>
<item>
<shape>
<gradient android:endColor="#ffffff" android:startColor="#efefef" android:type="linear" android:angle="90" />
<padding android:top="-20dp" />
</shape>
</item>
</layer-list>