Cleaning up drag state management.

When the drag is started, the UI automatically goes into spring loaded mode. On a successful
drop, it is the responsibility of the {@link DropTarget} to exit out of the spring loaded
mode. If the drop was cancelled for some reason, the UI will automatically exit out of this mode.

Bug: 34692289
Change-Id: Ic611739a43bb8d9279b587aaee3039326c143e8b

src/com/android/launcher3/ButtonDropTarget.java

@@ -229,7 +229,7 @@ public abstract class ButtonDropTarget extends TextView
             public void run() {
                 completeDrop(d);
                 mDropTargetBar.onDragEnd();
-                mLauncher.exitSpringLoadedDragModeDelayed(true, 0, null);
+                mLauncher.exitSpringLoadedDragMode(true, 0);
             }
         };
         dragLayer.animateView(d.dragView, from, to, scale, 1f, 1f, 0.1f, 0.1f,