AP_NavEKF: Don't reset the position measurement timeout if not aiding

When PV aiding is disabled, then the timeout time reference should not be reset becasue we want the position measurement timeout status to remain true the whole time the measurement is not being used.

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -1936,8 +1936,8 @@ void NavEKF::FuseVelPosNED()
             // use position data if healthy, timed out, or in constant position mode
             if (posHealth || posTimeout || constPosMode) {
                 posHealth = true;
-                // We don't reset the failed time if we are in constant position mode
-                if (!constPosMode) {
+                // only reset the failed time and do glitch timeout checks if we are doing full aiding
+                if (PV_AidingMode == AID_ABSOLUTE) {
                     posFailTime = imuSampleTime_ms;
                 }
                 // if timed out or outside the specified glitch radius, increment the offset applied to GPS data to compensate for large GPS position jumps