AP_NavEKF: Fix bug in pre-flight gyro bias logic

This reduces the noise on gyro bias estimates for copter and rover because
the increased process noise intended to aid initial bias estimation after
start is no longer tied to ground mode, which can toggle on and off for
those vehicle types, but is instead tied to the pre-arm static mode, which
is a more reliable indicator.

Also the scaler has been reduced to reduce noise on estimates pre-flight