Fixes the case where XAML tries to cast to a uint

Fixes #2384

Author: keveleigh

Assets/HoloToolkit/UX/Scripts/Keyboard.cs

@@ -253,7 +253,7 @@ protected override void Awake()
 
             // Setting the keyboardType to an undefined TouchScreenKeyboardType,
             // which prevents the MRTK keyboard from triggering the system keyboard itself.
-            InputField.keyboardType = (TouchScreenKeyboardType)(-1);
+            InputField.keyboardType = (TouchScreenKeyboardType)(int.MaxValue);
 
             // Keep keyboard deactivated until needed
             gameObject.SetActive(false);