AP_NavEKF2: Improve is flying check for non fly forward vehicles

Will enable use of EK3_MAG_TYPE = 0 for coptersAP_NavEKF2:

libraries/AP_NavEKF2/AP_NavEKF2_VehicleStatus.cpp

@@ -357,16 +357,24 @@ void NavEKF2_core::detectFlight()
             onGround = true;
         }
 
+        if (!onGround) {
             // If height has increased since exiting on-ground, then we definitely are flying
-        if (!onGround && ((stateStruct.position.z - posDownAtTakeoff) < -1.5f)) {
+            if ((stateStruct.position.z - posDownAtTakeoff) < -1.5f) {
                 inFlight = true;
             }
 
             // If rangefinder has increased since exiting on-ground, then we definitely are flying
-        if (!onGround && ((rangeDataNew.rng - rngAtStartOfFlight) > 0.5f)) {
+            if ((rangeDataNew.rng - rngAtStartOfFlight) > 0.5f) {
                 inFlight = true;
             }
 
+            // If more than 15 seconds armed since exiting on-ground, then we definitely are flying
+            if ((imuSampleTime_ms - timeAtArming_ms) > 15000) {
+                inFlight = true;
+            }
+
+        }
+
     }
 
     // store current on-ground  and in-air status for next time