services: autopilot-manager: Initialize self vars

* Add initial values for some member properties that aren't initialized
  in constructor but in setup

core/services/ardupilot_manager/autopilot_manager.py

@@ -43,6 +43,8 @@ def __init__(self) -> None:
         self.settings = Settings()
         self.settings.create_app_folders()
         self._current_board: Optional[FlightController] = None
+        self.should_be_running = False
+        self.mavlink_manager = None
 
         # Load settings and do the initial configuration
         if self.settings.load():