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/protos/launcher_atom.proto b/protos/launcher_atom.proto
index 036fb02..cac2d8f 100644
--- a/protos/launcher_atom.proto
+++ b/protos/launcher_atom.proto
@@ -26,6 +26,7 @@
     Task task = 2;
     Shortcut shortcut = 3;
     Widget widget = 4;
+    FolderIcon folder_icon = 9;
   }
   // When used for launch event, stores the global predictive rank
   optional int32 rank = 5;
@@ -92,6 +93,11 @@
   optional int32 index = 3;
 }
 
+// Represents folder in a closed state.
+message FolderIcon {
+  optional int32 cardinality = 1;
+}
+
 //////////////////////////////////////////////
 // Containers