athena: Use ActivityList where appropriate, and fix AppActivity tests.

athena/activity/activity_manager_impl.h

Index: athena/activity/activity_manager_impl.h
diff --git a/athena/activity/activity_manager_impl.h b/athena/activity/activity_manager_impl.h
index 9c1fb4e427b41c86317d026934544b75c4c13b0a..5989f1beb7c4073b32cb5118b2007b5d46a836c7 100644
--- a/athena/activity/activity_manager_impl.h
+++ b/athena/activity/activity_manager_impl.h
@@ -27,12 +27,17 @@ class ActivityManagerImpl : public ActivityManager,
   // ActivityManager:
   void AddActivity(Activity* activity) override;
   void RemoveActivity(Activity* activity) override;
+  ScopedActivity ReplaceActivity(Activity* to_replace,
+                                 Activity* replace_with) override;
   const ActivityList& GetActivityList() override;
   Activity* GetActivityForWindow(aura::Window* window) override;
   void AddObserver(ActivityManagerObserver* observer) override;
   void RemoveObserver(ActivityManagerObserver* observer) override;
 
  private:
+  void InitializeActivity(Activity* activity);
+  void UninitializeActivity(Activity* activity);
+
   // aura::WindowObserver:
   void OnWindowDestroying(aura::Window* window) override;