I am new to android and trying out some functions. I have an xml file that is called upon a button click:

```
<?xml version="1.0" encoding="utf-8"?>

<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_focused="true" android:drawable="@drawable/button_pressed"/>
<item android:state_pressed="true" android:drawable="@drawable/button_pressed" />
<item android:drawable="@drawable/btn_not_pressed" />  
</selector>

```

How do I edit this code so that when a button is pressed it remains in that state until it is clicked again, when it reverts back to the previous state?