Android round white button with drop shadow

Round_white_button_drop_shadow.xml

<?xml version="1.0" encoding="utf-8"?>

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <layer-list>
            <item>
                <shape android:shape="oval">
                    <gradient
                        android:startColor="#FF000000"
                        android:endColor="#00000000"

                        android:gradientRadius="31dp"
                        android:type="radial"
                        />
                </shape>
            </item>
            <item android:top="4dp" android:left="4dp" android:right="4dp" android:bottom="4dp">
            <shape android:shape="oval">
                <size android:width="55dp"
                    android:height="55dp"/>
                <solid android:color="@android:color/white" />
            </shape>
            </item>


        </layer-list>
    </item>

</selector>