res/drawable/circle_button.xml

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_pressed="true">

        <shape android:shape="oval">
            <size android:width="80dp" android:height="80dp"/>
            <stroke android:width="0.5dp" android:color="#FFBBBBBB" />
            <solid android:color="#BB888888" />
        </shape>

    </item>

    <item>
        <shape android:shape="oval">
            <size android:width="80dp" android:height="80dp"/>
            <stroke android:width="0.5dp" android:color="#FFBBBBBB" />
            <solid android:color="#BB444444" />
        </shape>
    </item>

</selector>