for others there are a solution for any API level , you can place a item on top of each other example :
<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android" >
<!-- my firt item with 4 corners radius(8dp)
-->
<item>
<shape>
<solid
android:angle="270.0"
android:color="#3D689A" />
<corners android:topLeftRadius="8dp" />
</shape>
</item>
<!-- my second item is on top right for a fake corner radius(0dp)
-->
<item
android:bottom="30dp"
android:left="50dp">
<shape>
<solid android:color="#5C83AF" />
</shape>
</item>
<!-- my third item is on bottom left for a fake corner radius(0dp)
-->
<item
android:right="50dp"
android:top="30dp">
<shape>
<solid android:color="#5C83AF" />
</shape>
</item>
</layer-list>
the result with light color to show you the three items :
the final result :
Best regards.