Implements LAUNCHER_ITEM_DROP_FOLDER_CREATED event.

When item is dropped on a existing item resulting in new folder creation, triggers LAUNCHER_ITEM_DROP_FOLDER_CREATED event with details of the destination package.
This change also introduces new FolderIcon item to launcher_atom.proto to represent folder icon.

Screencast and sample logs: https://docs.google.com/document/d/1CBP2yTcXdFhPdNG5ZmWFKSgd8mDbMevY-akVlUXPLDo/edit#bookmark=id.tmbucd1f44qp

Bug: 152978018

Change-Id: Ib4d343ba9075aa8853652f128457c4638541ec59