onStart method
Callback when this executor is started. Returns true if successfully started, false otherwise.
Implementation
@override
Future<bool> onStart() async {
  _subscription ??= SmartPhoneClientManager()
      .getStudyRuntime(deployment!.studyDeploymentId)
      ?.measurementsByType(configuration!.measureType)
      .listen((measurement) {
    if (configuration!.triggerCondition != null &&
        configuration!.triggerCondition!(measurement)) {
      onTrigger();
    }
  });
  return true;
}