land detector: move arming state into base class & set param when disarming

Before that, different modules (ekf2, commander & land detector) changed
params upon different events:
- ekf2 & commander set params after disarm
- land detector set params on land detected
If the 2 events were several 100ms appart, it led to 2 param saves, and
the latter param set could have been blocked by an ongoing save. And if
the land detector was blocked, it could lead to mag timeouts.

This patch makes all modules use the same event, thus only a single param
save will happen.

If we want to have guarantees to never block, we should introduce a
param_try_set() API method.

src/modules/land_detector/MulticopterLandDetector.h

@@ -49,7 +49,6 @@
 #include <uORB/topics/vehicle_local_position_setpoint.h>
 #include <uORB/topics/vehicle_attitude.h>
 #include <uORB/topics/actuator_controls.h>
-#include <uORB/topics/actuator_armed.h>
 #include <uORB/topics/actuator_controls.h>
 #include <uORB/topics/battery_status.h>
 #include <uORB/topics/parameter_update.h>
@@ -130,7 +129,6 @@ private:
 	int _vehicleLocalPositionSub;
 	int _vehicleLocalPositionSetpointSub;
 	int _actuatorsSub;
-	int _armingSub;
 	int _attitudeSub;
 	int _sensor_bias_sub;
 	int _vehicle_control_mode_sub;
@@ -139,7 +137,6 @@ private:
 	struct vehicle_local_position_s				_vehicleLocalPosition;
 	struct vehicle_local_position_setpoint_s	_vehicleLocalPositionSetpoint;
 	struct actuator_controls_s					_actuators;
-	struct actuator_armed_s						_arming;
 	struct vehicle_attitude_s					_vehicleAttitude;
 	struct sensor_bias_s					_sensors;
 	struct vehicle_control_mode_s				_control_mode;