Fixed Drag-and-Drop sample code

Fixed some erros in 'Responding to drag events' sample code on
http://developer.android.com/guide/topics/ui/drag-drop.html



Changed View.OnDragEventListener to View.OnDragListener,
deleted brace start('{') after DragEvent.ACTION_DRAG_ENTERED which
does not have matching ends, and removed 'break' on each case in
swich statement which causes 'Unreachable code' error.
Added return statement at the 'default' case, so that onDrag() method
can return value in all case.

Change-Id: I7d2335f5c405626e3f0cff275b52818762c84634
Signed-off-by: Taeho Kim <jyte82@gmail.com>