Moving drag-drop related code into a separate package.

This is per an earlier CR comment "we should probably move all this code to its own package (launcher3.dragndrop) in a separate cl".

I'm not moving DragSource because it's referred from gsa code.

Bug: 22609426
Change-Id: Ia7204dab99c0c395c66b77143a2d60411153f5f3

src/com/android/launcher3/CellLayout.java

@@ -405,7 +405,7 @@ public class CellLayout extends ViewGroup implements BubbleTextShadowHandler {
         mIsDragTarget = false;
     }
 
-    boolean isDragTarget() {
+    public boolean isDragTarget() {
         return mIsDragTarget;
     }
 
@@ -425,7 +425,7 @@ public class CellLayout extends ViewGroup implements BubbleTextShadowHandler {
         }
     }
 
-    boolean getIsDragOverlapping() {
+    public boolean getIsDragOverlapping() {
         return mIsDragOverlapping;
     }
 
@@ -2802,10 +2802,10 @@ public class CellLayout extends ViewGroup implements BubbleTextShadowHandler {
 
         // X coordinate of the view in the layout.
         @ViewDebug.ExportedProperty
-        int x;
+        public int x;
         // Y coordinate of the view in the layout.
         @ViewDebug.ExportedProperty
-        int y;
+        public int y;
 
         boolean dropped;