EKF: Use common timeout test for reporting deadreckoning

Use a parameter to set the time-out threshold
Use a class variable for the time-out flag
Clean up comments

EKF/estimator_interface.cpp

@@ -73,6 +73,7 @@ EstimatorInterface::EstimatorInterface():
 	_tas_test_ratio(0.0f),
 	_terr_test_ratio(0.0f),
 	_beta_test_ratio(0.0f),
+	_is_dead_reckoning(false),
 	_vibe_metrics{},
 	_time_last_imu(0),
 	_time_last_gps(0),