Add taskbar stashing feedforward, i.e. hint at upcoming stash/unstash

Also fix unstashing not working in 3P launchers by moving the unstash call from LauncherTaskbarUIController to TaskbarActivityContext

Test: long press taskbar background and stashed handle, watch it hint towards the new stashed/unstashed state respectively and then complete the animation when the long press is triggered
Fixes: 193926311
Fixes: 192926350
Change-Id: I0e538be9129bf5c600d07f360b8106d7077862ad

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

@@ -233,11 +233,6 @@ public class LauncherTaskbarUIController extends TaskbarUIController {
         }
     }
 
-    @Override
-    public boolean onLongPressToUnstashTaskbar() {
-        return mControllers.taskbarStashController.onLongPressToUnstashTaskbar();
-    }
-
     /**
      * @param ev MotionEvent in screen coordinates.
      * @return Whether any Taskbar item could handle the given MotionEvent if given the chance.