EKF: Rework nav validity reporting

Remove duplicate checking for dead reckoning and consolidate into a single function.
Use separate timers to check for start of dead reckoning and check when dead reckoning has been performed for too long for the nav solution to be valid.
Allow the timeout for validity reporting to be adjusted externally.
Separate external reporting of dead reckoning from internal checks.

EKF/estimator_interface.cpp

@@ -550,7 +550,7 @@ void EstimatorInterface::unallocate_buffers()
 bool EstimatorInterface::local_position_is_valid()
 {
 	// return true if we are not doing unconstrained free inertial navigation
-	return !inertial_dead_reckoning();
+	return !_deadreckon_time_exceeded;
 }
 
 void EstimatorInterface::print_status() {