import flash.desktop.NativeApplication;
import flash.events.KeyboardEvent;
import flash.ui.Keyboard;

NativeApplication.nativeApplication.addEventListener( KeyboardEvent.KEY_DOWN, handleKeyDown );

function handleKeyDown( e :KeyboardEvent ) :void
{
    switch( e.keyCode )
    {
        case Keyboard.BACK:
            NativeApplication.nativeApplication.exit();
            break;
 
        case Keyboard.SEARCH:
        case Keyboard.MENU:
            e.preventDefault();
            break;
    }
}

http://mobile.tutsplus.com/tutorials/android/build-a-gps-speedometer-getting-into-air-for-android/

Okay, we should now have an app that quits when the user presses the Back or Home keys rather than switching to the background and eating up battery.

AS3: Exiting Android App

ActionScript 3