Make LandingTargetEstimator a lib and run on work_queue

Currently only precision landing is using the
LandingTargetEstimator and it is a waste to have it
running in the background all the time. With this change
the estimator is only started once the precision landing
flight task is activated, and stopped when the flight
task is deactivated.

src/modules/flight_mode_manager/tasks/AutoPrecisionLanding/FlightTaskAutoPrecisionLanding.cpp

@@ -45,6 +45,16 @@
 
 static constexpr const char *LOST_TARGET_ERROR_MESSAGE = "Lost landing target while landing";
 
+FlightTaskAutoPrecisionLanding::FlightTaskAutoPrecisionLanding()
+{
+	_landing_target_estimator.Start();
+}
+
+FlightTaskAutoPrecisionLanding::~FlightTaskAutoPrecisionLanding()
+{
+	_landing_target_estimator.Stop();
+}
+
 bool FlightTaskAutoPrecisionLanding::activate(const trajectory_setpoint_s &last_setpoint)
 {
 	bool ret = FlightTask::activate(last_setpoint);