GCS_MAVLink: use att_pos_mocap covariance and fix extnav pos, ang err calculation

[chobitsfan]

1. Currently, the covariance field in ATT_POS_MOCAP is not used. This PR enables its usage, similar to what is already done in VISION_POSITION_ESTIMATE.
2. The current pose uncertainty calculation (posErr, angErr) appears to be incorrect. In a covariance matrix, the diagonal elements represent variances (i.e., squared quantities). Therefore, posErr should be computed as sqrt(covariance[0] + covariance[6] + covariance[11]).

flight log, ext nav data from kimera-vio
pr_fix_cov.zip
https://youtu.be/p1hQZSN-gU0

[peterbarker]

LGTM