code: @UiThread
      public boolean handleBack() {
          ThreadUtils.ensureMainThread();
  
          if (!backstack.isEmpty()) {
              //noinspection ConstantConditions
              if (backstack.peek().controller.handleBack()) {
                  return true;
              } else if (popCurrentController()) {
                  return true;
              }
          }
  
          return false;
      }
docs: This should be called by the host Activity when its onBackPressed method is called. The call will be forwarded
  to its top {@link Controller}. If that controller doesn't handle it, then it will be popped.
  
  @return Whether or not a back action was handled by the Router