Android: If you use your Android phone for streaming or video playback over HDMI, you’ve probably run into the problem of your device’s screen staying on unnecessarily. This can run down the battery, and there’s no need to see the image on both screens. Screen Standby fixes that problem.
Screen Standby simply sets your screens brightness to zero when you are streaming/playing video over HDMI. It’s a simple trick but solves an annoying problem for those of us who use our Androids as a media centre. Once activated. you can still see the image you’re streaming on your TV (or wherever), but your screen will be off. To get things back to normal, all you have to do is lock and unlock your device.
Screen Standby is free to download, but it does require rooting your device.
Screen Standby [XDA Developers]