Implement device screen off while mirroring

Add two shortcuts:
 - Ctrl+o to turn the device screen off while mirroring
 - Ctrl+Shift+o to turn it back on

On power on (either via the POWER key or BACK while screen is off), both
the device screen and the mirror are turned on.

<https://github.com/Genymobile/scrcpy/issues/175>

server/src/main/java/com/genymobile/scrcpy/Controller.java

@@ -97,6 +97,9 @@ public class Controller {
             case ControlMessage.TYPE_SET_CLIPBOARD:
                 device.setClipboardText(msg.getText());
                 break;
+            case ControlMessage.TYPE_SET_SCREEN_POWER_MODE:
+                device.setScreenPowerMode(msg.getAction());
+                break;
             default:
                 // do nothing
         }