FlightTask: Fix ekf2 reset race condition during task switch (#14692)

* FlightTask: Fix ekf2 reset race condition during task switch

During a loss of GPS data when using GPS as primary height source,
the height is reset to baro and the local position gets invalid at the
same time. This triggers a switch to altitude flight task and a setpoint
reset.
This combination of events had the effect to ignore the height reset,
the large sudden height error could create an abrupt change of altitude
or even a crash.
The ekf2 reset is now done at the beginning of each update call.

src/lib/flight_tasks/tasks/ManualAltitude/FlightTaskManualAltitude.cpp

@@ -361,10 +361,11 @@ bool FlightTaskManualAltitude::_checkTakeoff()
 
 bool FlightTaskManualAltitude::update()
 {
+	bool ret = FlightTaskManual::update();
 	_updateConstraintsFromEstimator();
 	_scaleSticks();
 	_updateSetpoints();
 	_constraints.want_takeoff = _checkTakeoff();
 
-	return true;
+	return ret;
 }