AP_NavEKF3: do not look at takeoff for zero velocity fusion

[andyp1per]

Summary

Fixes the plane dead reckoning tests, by not considering takeoff as a potential time to not fuse.

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Description

This is a simpler fix than #32986 - it fixes the dead reckoning tests in an obvious way. It does mean that zero velocity fusion occurs when armed and on the ground, but t hat was true before anyway

[tridge]

@andyp1per I think we merge this, but I'd like to consider if we should disable the zero vel fusion when fly-forward flag is set

[rmackay9]

We discovered during the investigation of logs from PeterB's revert PR #33128 that plane, with no GPS, will incorrectly determine that it is not flying once its air speed drops close to zero. We had a lengthy discussion about the difficulties of determining if a fixed wing is flying or not.

[tpwrules]

What about the similar onGroundNotFlying calculation below?