Use LauncherCallbacks model instead of method overrides

-> When extending the Launcher Activity, instead of overriding
   public and protected methods, create a proper interface
-> This helps define the interface when extending Launcher
   more formally and more clearly

Change-Id: Ib38e8a376b2242d4078bf6856bb145f5b5f0da80

src/com/android/launcher3/Workspace.java

@@ -1267,7 +1267,6 @@ public class Workspace extends SmoothPagedView
             mCustomContentShowing = false;
             if (mCustomContentCallbacks != null) {
                 mCustomContentCallbacks.onHide();
-                mLauncher.resetQSBScroll();
                 mLauncher.updateVoiceButtonProxyVisible(false);
             }
         }