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/TaskbarView.java

@@ -222,6 +222,15 @@ public class TaskbarView extends FrameLayout implements FolderIcon.FolderIconPar
         return super.dispatchTouchEvent(ev);
     }
 
+    @Override
+    public boolean onTouchEvent(MotionEvent event) {
+        if (!mTouchEnabled) {
+            return true;
+        }
+        mControllerCallbacks.onTouchEvent(event);
+        return super.onTouchEvent(event);
+    }
+
     public void setTouchesEnabled(boolean touchEnabled) {
         this.mTouchEnabled = touchEnabled;
     }