Draw folder leave-behind when opening folder in taskbar

Previously there was only support for CellLayout to draw the
leave-behind. Added FolderIconParent interface so any parent can
draw the leave-behind, and implement that by TaskbarView.

Test: Open folder in Taskber, ensure leave-behind draws

Bug: 171917176
Change-Id: I08d1d939a34f971f893994fe05cac972d165ef53