Add Mojo constant for "ash" and replace hardcoded strings

ash/public/interfaces/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("interfaces") {
   sources = [
     "accelerator_controller.mojom",
     "ash_window_type.mojom",
     "cast_config.mojom",
+    "constants.mojom",
     "locale.mojom",
     "media.mojom",
     "new_window.mojom",
     "session_controller.mojom",
     "shelf.mojom",
     "shutdown.mojom",
     "system_tray.mojom",
     "touch_view.mojom",
     "update.mojom",
     "volume.mojom",
     "vpn_list.mojom",
     "wallpaper.mojom",
   ]
 
   public_deps = [
     "//components/signin/public/interfaces",
     "//skia/public/interfaces",
   ]
 }