const mouse key device id

Runtime/engine/input_utils.cs

 using UnityEngine.EventSystems;
 
 namespace Unity.UIWidgets.engine {
-    static class InputUtils {
+    public static class InputUtils {
+
+        public const int MouseLeftKeyDevice = preservedKeyNum;
+        public const int MouseRightKeyDevice = preservedKeyNum + 1;
 
         public static PointerDeviceKind getPointerDeviceKind(PointerEventData eventData) {
             return isTouchEvent(eventData) ? PointerDeviceKind.touch : PointerDeviceKind.mouse;