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