Actually call LauncherTransitionable.onLauncherTransitionStep().

Previously, it was only called at the start and end of the transition;
now it is called as the animation interpolates. Specifically, a dummy
ValueAnimator is played alongside the transition animation and calls
dispatchOnLauncherTransitionStep() as it goes.

One place where this is important is in Workspace, where
mTransitionProgress is used to determine things like whether the
workspace should accept a drop - hence the bug that caused apps dragged
from All Apps to vanish when dropped before the transition ended.

Bug: 24215358
Change-Id: I32cd633c53557305caf84e87c9a4d4f07eef2223

src/com/android/launcher3/Launcher.java

@@ -3824,6 +3824,7 @@ public class Launcher extends Activity
      *
      * Implementation of the method from LauncherModel.Callbacks.
      */
+    @Override
     public void bindItems(final ArrayList<ItemInfo> shortcuts, final int start, final int end,
                           final boolean forceAnimateIcons) {
         Runnable r = new Runnable() {