这是一个Google issue：
https://code.google.com/p/android/issues/detail?id=63570 comment#2
https://www.google.com/?gws_rd=ssl#hl=zh-CN&q=android+4.4+FLAG_ACTIVITY_REORDER_TO_FRONT
http://stackoverflow.com/questions/20695522/puzzling-behavior-with-reorder-to-front