Improve EV height starting condition

[bresch]

follows https://github.com/PX4/PX4-Autopilot/issues/16489

The EV height starting condition only checks if the gathered data is not too old : https://github.com/PX4/PX4-ECL/blob/b919f3ddf41ab422d09af91eaea2b2f9729fab13/EKF/control.cpp#L1037) In some cases, it can be that the EV system provides wrong data; in that case, it should be rejected and EV height should not start (or re-start).

FYI @cimbar

[antonio-sc66]

Checking the Z variance should help (posVar(2) of the extVisionSample struct) as one of the common problems when the EV systems are providing wrong data is that the position or velocity values tend to vary a lot and rapidly. The problem with this method is that there are EV systems that don't fill the covariance matrix, in those cases the posVar will have the EKF2_EVP_NOISE and the filtering wont apply.

https://github.com/PX4/PX4-Autopilot/blob/f13c3a7d44c58f69c77f9e4f82cf3d6a41ec9551/src/modules/ekf2/EKF2.cpp#L1294-L1303