EKF3: improve recovery from OpticalFlow->GPS

[rmackay9]

EKF3's velocity estimates become odd if both GPS and optical flow are enabled and it then loses GPS. Below is a screen shot (from SITL) of the vehicle's velocity estimate and we see it is spiking up and down very rapidly. The vehicle does not fly as expected especially when GPS is restored (it flies forward quickly for a short time)

To reproduce the issue in SITL do this:

• param set EK3_ENABLE 1
• param set EK2_ENABLE 0
• param set AHRS_EKF_TYPE 3 (to enable EKF3)
• follow instructions to enable OpticalFlow in SITL
• param set EK3_GPS_TYPE 0 (to keep using GPS)
• follow instructions to enable RangeFinder in SITL
• loiter
• arm throttle
• takeoff
• rc 3 1800 (to climb to 10m)
• rc 3 1500 (to level out)
• rc 2 1300 (to fly forward)
• param set GPS_TYPE 0 (to disable GPS, wait 10 seconds)
• param set GPS_TYPE 1 (to re-enable GPS)

[rmackay9]

The PR linked above plus this latest PR #15670 have resolved this issue in master