FirstActivity啓動SecondActivity,生命流程
FirstActivity onCreate-onstart-OnResume-onPause, SecondActivity onCreate-onstart-OnResume 調用SecondActivity以後SecondActivity完全展示了出來,此時調用FirstActivity的onStop方法.同理 當點擊返回鍵回到FirstActivity的時候.
調用FisrtActivity的 onStart- OnResume-後調用Second的onStop和onDestory().