关于Activity的singleInstance特性
MainActivity --启动--> ActivityA（singleInstance）

Task 2 Back Stack in the foreground:
ActivityA

Task 1 Back Stack:
MainActivity


MainActivity --启动--> ActivityA（singleInstance）--启动--> ActivityB

Task 1 Back Stack in the foreground:
ActivityB
MainActivity

Task 2 Back Stack:
ActivityA


MainActivity --启动--> ActivityA（singleInstance）--启动--> ActivityB ----> ActivityA

Task 2 Back Stack in the foreground:
ActivityA

Task 1 Back Stack:
ActivityB
MainActivity


MainActivity --启动--> ActivityA（singleInstance）--启动--> ActivityB ----> ActivityA ----> ActivityB

Task 1 Back Stack in the foreground:
ActivityB
ActivityB
MainActivity

Task 2 Back Stack:
ActivityA

用以下调试命令查看堆栈：
adb shell dumpsys activity activities