Here's a simple circle_background.xml for pre-material:
<?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="oval">
<solid android:color="@color/color_accent_dark" />
</shape>
</item>
<item>
<shape android:shape="oval">
<solid android:color="@color/color_accent" />
</shape>
</item>
</selector>
You can use with the attribute 'android:background="@drawable/circle_background"
in your button's layout definition