mash: Add SystemTrayClient interface, use to show date settings

ash/mus/manifest.json

 {
   "manifest_version":  1,
   "name": "mojo:ash",
   "display_name": "Ash Window Manager and Shell",
   "capabilities": {
     "provided": {
       "mus:window_manager" : [ "ui::mojom::AcceleratorRegistrar" ]
     },
     "required": {
       "*": { "classes": [ "accessibility", "app" ] },
       "exe:chrome" : {
         "interfaces": [
+          "ash::mojom::SystemTrayClient",
           "ash::mojom::WallpaperManager",
           "app_list::mojom::AppListPresenter"
         ]
       },
       "mojo:mash_session": {
         "interfaces": [ "mash::session::mojom::Session" ]
       },
       "mojo:ui": {
         "interfaces": [
           "display::mojom::DisplayController",
           "ui::mojom::AccessibilityManager",
           "ui::mojom::WindowManagerWindowTreeFactory"
         ]
       },
       "mojo:touch_hud": {
         "interfaces": [
           "mash::mojom::Launchable"
         ]
       }
     }
   }
 }