API level < 21

radio_group_bg_last_child.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:state_pressed="true" >
        <shape android:shape="rectangle">
            <solid android:color="@color/fill_color_when_pressed" />
            <corners android:topRightRadius="@dimen/corner_radius" android:bottomRightRadius="@dimen/corner_radius"/>
        </shape>
    </item>

    <item android:state_checked="true" >
        <shape android:shape="rectangle">
            <solid android:color="@color/fill_color_when_active" />
            <corners android:topRightRadius="@dimen/corner_radius" android:bottomRightRadius="@dimen/corner_radius"/>
        </shape>
    </item>

    <item android:drawable="@android:color/transparent"/>

</selector>