ekf2: add simple zero velocity update when vehicle at rest (#19149)

- further decreases initial tilt alignment time (now down to 2.5 seconds if still) and improves initial bias estimates

src/modules/ekf2/test/test_EKF_basics.cpp

@@ -53,7 +53,12 @@ public:
 	// Setup the Ekf with synthetic measurements
 	void SetUp() override
 	{
+		// run briefly to init, then manually set in air and at rest (default for a real vehicle)
 		_ekf->init(0);
+		_sensor_simulator.runSeconds(0.1);
+		_ekf->set_in_air_status(false);
+		_ekf->set_vehicle_at_rest(true);
+
 		_sensor_simulator.runSeconds(_init_duration_s);
 	}