res/drawable/keyboard_key_background.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2014 Google Inc. All Rights Reserved. -->
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_pressed="true" >
        <shape android:shape="oval">
            <solid android:color="@color/ime_background_pressed" />
            <corners android:radius="2dp" />
        </shape>
    </item>
    <item android:drawable="@color/transparent_black" />
</selector>