Remove background scrim when there are no top insets.

ie. In portrait multiwindow mode, the launcher is always on the
bottom so we can remove the background scrim that is meant to
protect the status bar contents.

Bug: 36397512
Change-Id: Ie9655b0604b5bebafdfb4153b370bb655129051c

src/com/android/launcher3/dragndrop/DragLayer.java

@@ -448,6 +448,12 @@ public class DragLayer extends InsettableFrameLayout {
         return isContainerOpen || mDragController.dispatchUnhandledMove(focused, direction);
     }
 
+    @Override
+    public void setInsets(Rect insets) {
+        super.setInsets(insets);
+        setBackgroundResource(insets.top == 0 ? 0 : R.drawable.workspace_bg);
+    }
+
     @Override
     public LayoutParams generateLayoutParams(AttributeSet attrs) {
         return new LayoutParams(getContext(), attrs);