How to do an outer glow via xml on button?

Question

I have a customized button with rounded corners, I put some shadow for when it's pressed but I want to do an outer shadow just to the bottom part of the button, I am making the drawable via xml, so if the glow could be that way would be great. These are the relevant parts of the code:

button_pressed_shadows.xml

<?xml version="1.0" encoding="UTF-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android" >
    <item>
        <shape android:shape="rectangle" >
           <gradient 
               android:startColor="@color/black_overlay"
               android:endColor="@color/btn_login"
               android:angle="270"/>
           <corners android:radius="4dip" />
        </shape>
    </item>

    <item               
        android:top="2px">
        <shape android:shape="rectangle"> 
            <solid android:color="@color/btn_login"/>
            <corners android:radius="4dip" />
         </shape>
    </item>
</layer-list>

style_login.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_pressed="true"
          android:drawable="@drawable/button_pressed_shadows" /> <!-- pressed -->
    <item android:state_focused="true"
          android:drawable="@drawable/button_focused" /> <!-- focused -->
    <item android:state_hovered="true"
          android:drawable="@drawable/button_focused" /> <!-- hovered -->
    <item android:drawable="@drawable/button_login" /> <!-- default -->
</selector>

Answer 1

Since I have the button design on photoshop, I made a 9 patch image with it, and put it on the style selector, everything went fine, think is the better (easiest) way.