Move mus to //services/ui

services/ui/ws/mus_ws_unittests_app_manifest.json

 {
   "manifest_version":  1,
   "name": "mojo:mus_ws_unittests_app",
   "display_name": "Mus Window Server Unittests",
   "capabilities": {
     "required": {
       "*": { "classes": [ "app" ] },
       "mojo:mus_ws_unittests_app": {
         "interfaces": [ "mus::mojom::WindowTreeClient" ]
       },
       "mojo:mus": {
         "interfaces":  [ "mus::mojom::WindowTreeHostFactory" ]
       }
     }
   }
 }