Add shutdown consumers

Significant changes:
- Getters were made to return const strings.
- The object_path property of the legacy app handler was made const.
- DLT log levels changed in some places.
- in la_handler_service_handle_consumer_shutdown_finish, logic has been
  changed to not compare the value of "result" unless there were no
  errors, because boot_manager_call_stop_finish does not necessarily
  always give a valid pointer.