FW Position Control: improve auto takeoff behavior without launch detection

[sfuhrer]

Solved Problem

1. when the airspeed measurement is negative, the input into the filter is TRIM airspeed, which results in a growing airspeed_filtered value even though the actual airspeed is still 0, plus it made the underspeed flag toggle between on and off.
2. the airspeed underspeed logic shouldn't be active during takeoff (anyway not able to pitch down due to FW_TKO_PITCH_MIN)

Solution

1. leave the airspeed validity logic to the airspeed validator, and use measurement in the FW position controller as long as finite.
2. Disable airspeed underspeed detection logic during auto takeoff, as we already do for the landing for example.

Changelog Entry

For release notes:

Improvement: FW Position Control: improve auto takeoff behavior without launch detection

Test coverage

SITL tested.