A的启动：onCreate->onStart->onResume
当A去启动B，首先A调用onPause，然后B调用onCreate->onStart->onResume
然后A会调用onStop
当按下返回键，B调用onPause，然后A调用onRestart->onStart->onResume
B调用onStop->onDestory
当再次按下返回键，A调用onPause->onStop->onDestory