Allow subclassing Launcher

Changes included:

- Make Launcher not final

- Add some new methods as hooks for subclasses

- Allow Launcher to intercept Workspace touch events

Change-Id: I9a2b628c853394a3b237a384336e1b9e5c2382ec

src/com/android/launcher3/Workspace.java

@@ -663,6 +663,11 @@ public class Workspace extends SmoothPagedView
                 }
             }
         }
+
+        if (mLauncher != null && mLauncher.onTouch(this, ev)) {
+            return true;
+        }
+
         return super.onInterceptTouchEvent(ev);
     }