Remove on drag listener after drag ended

Remove the on drag listener for the drag layer
in the taskbar after ACTION_DRAG_ENDED is received

This is an additional refactoring discussed
in ag/20490352

Bug: 258850827

Test: manually reproduced the steps in the bug
Change-Id: Ibe207001f41d3ba97a8acaf17d633cbd6682fafd

quickstep/src/com/android/launcher3/taskbar/TaskbarDragController.java

@@ -427,6 +427,8 @@ public class TaskbarDragController extends DragController<BaseTaskbarContext> im
                         // This will take care of calling maybeOnDragEnd() after the animation
                         animateGlobalDragViewToOriginalPosition(btv, dragEvent);
                     }
+                    mActivity.getDragLayer().setOnDragListener(null);
+
                     return true;
             }
             return false;