Decoupling some drag and drop code handling

> Removing some special checks around accessibility drag
> Unifying folder alarm code path for accessible and normal DnD
> Maintaining mDragStartTime inside the dragDriver instead of the callers
> Simplifying some accessibility callbacks

Future cl will create a Accessibility DragDriver and unify it with
other DnD flow

Change-Id: I1919ef218de0174678110f271b450bcb9aaf4a5c

src/com/android/launcher3/DropTarget.java

@@ -59,9 +59,6 @@ public interface DropTarget {
         /** Where the drag originated */
         public DragSource dragSource = null;
 
-        /** The object is part of an accessible drag operation */
-        public boolean accessibleDrag;
-
         /** Indicates that the drag operation was cancelled */
         public boolean cancelled = false;