AP_NavEKF2: use AHRS likely flying state

this sets inFlight when AHRS has indicated flying for 5s

libraries/AP_NavEKF2/AP_NavEKF2_VehicleStatus.cpp

@@ -368,11 +368,11 @@ void NavEKF2_core::detectFlight()
                 inFlight = true;
             }
 
-            // If more than 15 seconds armed since exiting on-ground, then we definitely are flying
-            if ((imuSampleTime_ms - timeAtArming_ms) > 15000) {
+            // If more than 5 seconds since likely_flying was set
+            // true, then set inFlight true
+            if (_ahrs->get_time_flying_ms() > 5000) {
                 inFlight = true;
             }
-
         }
 
     }