code: private Activity getLastActivityInner(){
          synchronized (LOCK_LASTACTIVITIES) {
              if (curActivities.size() > 0) {
                  return curActivities.get(curActivities.size()-1);
              } else {
                  return null;
              }
          }
      }
docs: 获取最新的activity，如果没有则返回null | Gets the latest activity and returns null if not
  @return 最新的activity | Latest activity