Fix add extra empty screen logic

The bug occurred when there are two widgets in a CellLayout. When
dragging one of the widget, ShortcutAndWidgetContainer#getChildCount
returns 1 rather than 2 because the LauncherAppWidgetHostView of the
widget in drag has been detached from its parent and reattached to
DragView (introduced in ag/14234627).

Test: Place only two widgets on the last page of the home screen. Drag
      one widget. The new page indicator on the right is shown.
Bug: 188494523
Change-Id: I8255dd1b72ad7f18f4cb35a9e14ac8e61804980c