mus: Introduce high-contrast mode in mus.

services/ui/manifest.json

 {
   "manifest_version":  1,
   "name": "mojo:ui",
   "display_name": "UI Service",
   "capabilities": {
     "provided": {
       // A collection of interfaces needed by a generic client of mus.
       // Additional interfaces may be requested a-la-carte.
       "app": [
         "ui::mojom::Clipboard",
         "ui::mojom::DisplayManager",
         "ui::mojom::Gpu",
         "ui::mojom::GpuService",
         "ui::mojom::InputDeviceServer",
         "ui::mojom::WindowTreeFactory"
       ],
+      "accessibility": [
+        "ui::mojom::AccessibilityManager"
+      ],
       "test": [
         "ui::mojom::WindowServerTest"
       ]
     },
     "required": {
       "*": { "classes": [ "app" ] },
       "mojo:shell": { "classes": [ "shell:all_users", "shell:explicit_class" ] }
     }
   }
 }