EXP-1538 FIX -- New tags shown for items in subfolders in received items panel...

EXP-1538 FIX -- New tags shown for items in subfolders in received items panel which remain after minimizing parent folder

* "new" tag determination for LLInboxFolderViewItem is now done on "addToFolder" rather than at construction time
  to avoid computing "new" for items not directly in the top level folder.