ekf2: move aiding start details in dedicated function

When an aiding source needs to be started, simply call the corresponding
starting function and any required reset is handled in there.

src/modules/ekf2/test/test_EKF_gps.cpp

@@ -152,6 +152,7 @@ TEST_F(EkfGpsTest, gpsHgtToBaroFallback)
 	_sensor_simulator.runSeconds(1);
 	EXPECT_TRUE(_ekf_wrapper.isIntendingGpsHeightFusion());
 	EXPECT_TRUE(_ekf_wrapper.isIntendingFlowFusion());
+	EXPECT_FALSE(_ekf_wrapper.isIntendingBaroHeightFusion());
 
 	// WHEN: stopping GPS fusion
 	_sensor_simulator.stopGps();