Fix taskbar folder leave-behind not being drawn

I guess FrameLayout has willNotDraw=true by default, so we need to set it to false in order to get onDraw().

Test: open a folder on taskbar
Bug: 197866264
Change-Id: Ia521bc3da28527b9c3f1fa7bcdaf5d9ffb7feccf

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

@@ -90,6 +90,9 @@ public class TaskbarView extends FrameLayout implements FolderIcon.FolderIconPar
         // We layout the icons to be of mIconTouchSize in width and height
         mItemMarginLeftRight = actualMargin - (mIconTouchSize - actualIconSize) / 2;
         mItemPadding = (mIconTouchSize - actualIconSize) / 2;
+
+        // Needed to draw folder leave-behind when opening one.
+        setWillNotDraw(false);
     }
 
     protected void init(TaskbarViewController.TaskbarViewCallbacks callbacks) {