Cleanup and refactoring in relation to drag and drop

-> folder creation, adding to folders, reordering, switching pages
-> ensuring parity of dragEnter and dragExit in cell layouts and workspace
   (neither were consistently getting an even number of each)
-> actually enforced above with exceptions -- probably want to
   take these out, but maybe we can leave them in as warnings
-> fixed bug with mapping points to hotseat
-> fixes other bugs with drag and drop

Change-Id: I564568f810f2784d122ec6135012b67c2e8e7551

src/com/android/launcher2/DropTarget.java

@@ -16,6 +16,7 @@
 
 package com.android.launcher2;
 
+import android.content.Context;
 import android.graphics.PointF;
 import android.graphics.Rect;
 
@@ -63,6 +64,43 @@ public interface DropTarget {
         }
     }
 
+    public static class DragEnforcer implements DragController.DragListener {
+        int dragParity = 0;
+
+        public DragEnforcer(Context context) {
+            Launcher launcher = (Launcher) context;
+            launcher.getDragController().addDragListener(this);
+        }
+
+        void onDragEnter() {
+            dragParity++;
+            if (dragParity != 1) {
+                throw new RuntimeException("onDragEnter: Drag contract violated: " + dragParity);
+            }
+        }
+
+        void onDragExit() {
+            dragParity--;
+            if (dragParity != 0) {
+                throw new RuntimeException("onDragExit: Drag contract violated: " + dragParity);
+            }
+        }
+
+        @Override
+        public void onDragStart(DragSource source, Object info, int dragAction) {
+            if (dragParity != 0) {
+                throw new RuntimeException("onDragEnter: Drag contract violated: " + dragParity);
+            }
+        }
+
+        @Override
+        public void onDragEnd() {
+            if (dragParity != 0) {
+                throw new RuntimeException("onDragExit: Drag contract violated: " + dragParity);
+            }
+        }
+    }
+
     /**
      * Used to temporarily disable certain drop targets
      *