How to set a different background for function key in xml on keyboard?

I am developing a keyboard application for Android. I tried to set different background for normal key and function key but it didn't work:

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

    <!-- Functional keys. -->

    <item android:state_single="true" android:state_pressed="true"
          android:drawable="@drawable/btn_keyboard_special" />
    <item android:state_single="true"
          android:drawable="@drawable/btn_keyboard_special" />

    <!-- Toggle keys. Use checkable/checked state.   -->

    <item android:state_checkable="true" android:state_checked="true" android:state_pressed="true"
          android:drawable="@drawable/btn_keyboard_key_dark_pressed_on" />
    <item android:state_checkable="true" android:state_pressed="true"
          android:drawable="@drawable/btn_keyboard_key_dark_pressed_off" />
    <item android:state_checkable="true" android:state_checked="true"
          android:drawable="@drawable/btn_keyboard_key_dark_normal_on" />
    <item android:state_checkable="true"
          android:drawable="@drawable/btn_keyboard_key_dark_normal_off" /> 

    <!-- Normal keys -->

    <item android:state_pressed="true"
          android:drawable="@drawable/glow" />
    <item android:drawable="@drawable/btn_keyboard_key_light_normal" />
</selector>