[PhotoCellNode]Update formatting of layoutSpec to a flat structure. (#2732)
[PhotoCellNode]Update formatting of layoutSpec to a flat structure. (#2732)

* [PhotoCellNode]Update formatting of layoutSpec to a flat structure.

* [PhotoCellNode] Adding a flag to switch between flat and tree like structure of layout specs.

JetZou/AsyncDisplayKit,flovouin/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,lappp9/AsyncDisplayKit,flovouin/AsyncDisplayKit,flovouin/AsyncDisplayKit,maicki/AsyncDisplayKit,flovouin/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,rmls/AsyncDisplayKit,JetZou/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,rmls/AsyncDisplayKit,romyilano/AsyncDisplayKit,maicki/AsyncDisplayKit,harryworld/AsyncDisplayKit,JetZou/AsyncDisplayKit,romyilano/AsyncDisplayKit,JetZou/AsyncDisplayKit,harryworld/AsyncDisplayKit,romyilano/AsyncDisplayKit,maicki/AsyncDisplayKit,maicki/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,rmls/AsyncDisplayKit,flovouin/AsyncDisplayKit,romyilano/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,lappp9/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,maicki/AsyncDisplayKit,JetZou/AsyncDisplayKit,harryworld/AsyncDisplayKit,rmls/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,harryworld/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,harryworld/AsyncDisplayKit,lappp9/AsyncDisplayKit,lappp9/AsyncDisplayKit,rmls/AsyncDisplayKit