1- Add this permission on the descriptor files (HelloWorld-app.xml):
<uses-permission android:name="android.permission.WAKE_LOCK" />
2- Add this in your Main AS project:
NativeApplication.nativeApplication.systemIdleMode = SystemIdleMode.KEEP_AWAKE;
I thank you but this only seems to work for android and Iphone app. Mine is Windows desktop application and it doesn'work in this case.
Maybe in a future version...
Here's what I found from the docs:
For AIR content running in full-screen mode, the system screen saver and power-saving options are disabled during play until either the video input stops or the user exits full-screen mode
There's also been at least one feature request for this on ideas.adobe.com. I'd encourage you to cast you vote and let others know that might also benefit.