mus: Introduce high-contrast mode in mus.

ash/sysui/manifest.json

 {
   "manifest_version":  1,
   "name": "mojo:ash_sysui",
   "display_name": "System UI",
   "capabilities": {
     "required": {
-      "*": { "classes": [ "app" ] },
+      "*": { "classes": [ "accessibility", "app" ] },
       "mojo:ash": {
         "interfaces": [
           "ash::mojom::ShelfLayout",
           "ash::mojom::UserWindowController"
         ]
       },
       "exe:chrome" : {
         "interfaces": [ "ash::sysui::mojom::WallpaperManager" ],
         "classes": [ "app_list:presenter" ]
       }
     }
   }
 }