当在activity的onCreate()中调用了startService()，不是执行到activity的onCreate()的时候就去启动service，
而是一个组件启动完了再去启动另一个组件
运行日志：
activity --- 构造方法
activity --- onCreate()
activity --- onStart()
activity --- onResume()
service --- 构造方法
service --- onCreate()
service --- onStartCommand()
当在MainActivity的onCreate()中调用startActivity()启动Activity2
运行日志：
D/MainActivity------: onCreate:
D/MainActivity------: onStart:
D/MainActivity------: onResume:
D/MainActivity------: onPause:
D/Activity2------: onCreate:
D/Activity2------: onStart:
D/Activity2------: onResume:
D/MainActivity------: onStop:按下一次返回键----------------------------------
D/Activity2------: onPause:
D/MainActivity------: onStart:
D/MainActivity------: onResume:
D/Activity2------: onStop:
D/Activity2------: onDestroy:
当在MainActivity的onCreate()中add一个Fragment
D/MainActivity------: onCreate:
D/LeftFragment--------: onAttach:
D/LeftFragment--------: onCreate:
D/LeftFragment--------: onCreateView:
D/LeftFragment--------: onActivityCreated:
D/LeftFragment--------: onStart:
D/MainActivity------: onStart:
D/MainActivity------: onResume:
D/LeftFragment--------: onResume:
按下一次返回键-----------------------------D/LeftFragment--------: onPause:
D/MainActivity------: onPause:
D/LeftFragment--------: onStop:
D/MainActivity------: onStop:
D/LeftFragment--------: onDestroyView:
D/LeftFragment--------: onDestroy:
D/LeftFragment--------: onDetach:
D/MainActivity------: onDestroy: