Make setScreenPowerMode() method static

Its implementation does not use the instance at all.

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

@@ -116,7 +116,7 @@ public class Controller {
             case ControlMessage.TYPE_SET_SCREEN_POWER_MODE:
                 if (device.supportsInputEvents()) {
                     int mode = msg.getAction();
-                    boolean setPowerModeOk = device.setScreenPowerMode(mode);
+                    boolean setPowerModeOk = Device.setScreenPowerMode(mode);
                     if (setPowerModeOk) {
                         Ln.i("Device screen turned " + (mode == Device.POWER_MODE_OFF ? "off" : "on"));
                     }

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

@@ -230,7 +230,7 @@ public final class Device {
     /**
      * @param mode one of the {@code SCREEN_POWER_MODE_*} constants
      */
-    public boolean setScreenPowerMode(int mode) {
+    public static boolean setScreenPowerMode(int mode) {
         IBinder d = SurfaceControl.getBuiltInDisplay();
         if (d == null) {
             Ln.e("Could not get built-in display");