Make UserActivityDetector a singleton

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 51c912b6b36e3a457a0b256514999265ad5b718a..6ab3941ce725c23fcff0e1c3754d3692287b2e24 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -368,9 +368,6 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   MruWindowTracker* mru_window_tracker() {
     return mru_window_tracker_.get();
   }
-  ::wm::UserActivityDetector* user_activity_detector() {
-    return user_activity_detector_.get();
-  }
   VideoDetector* video_detector() {
     return video_detector_.get();
   }