onStart method
Callback from the app when this task is to be started.
Implementation
@mustCallSuper
void onStart() {
  // initialize the background task which holds any measures added to the app task
  backgroundTaskExecutor.initialize(
    task.backgroundTask,
    _executor.deployment,
  );
  // // add the events from the background executor to the overall stream of events
  // _executor.addExecutor(backgroundTaskExecutor);
  state = UserTaskState.started;
}