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启动MainActivity
MainActivity: onCreate
MainActivity: onStart
MainActivity: onResume从MainActivity启动SecondActivity
MainActivity: onPause//另一个Activity插入到MainActivity前面
SecondActivity: onCreate
SecondActivity: onStart
SecondActivity: onResume
MainActivity: onSaveInstanceState
MainActivity: onStop//MainActivity不可见点击Home键
SecondActivity: onPause
SecondActivity: onSaveInstanceState
SecondActivity: onStop点击图标启动
SecondActivity: onRestart
SecondActivity: onStart
SecondActivity: onResume点击返回键
SecondActivity: onPause
MainActivity: onRestart
MainActivity: onStart
MainActivity: onResume
SecondActivity: onStop
SecondActivity: onDestroy