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
diff --git a/src/com/android/launcher3/DropTarget.java b/src/com/android/launcher3/DropTarget.java
index 6cb803a..0b0983c 100644
--- a/src/com/android/launcher3/DropTarget.java
+++ b/src/com/android/launcher3/DropTarget.java
@@ -78,7 +78,7 @@
         public DraggableView originalView = null;
 
         /** Used for matching DROP event with its corresponding DRAG event on the server side. */
-        final InstanceId mLogInstanceId =
+        public final InstanceId logInstanceId =
                 new InstanceIdSequence(1 << 20 /*InstanceId.INSTANCE_ID_MAX*/)
                     .newInstanceId();