AP_NavEKF: Reduce time required to recover from GPS timeouts

This reduces the time required to recover from bad inertial data for planes and also reduces the time required to recover when GPS is regained after an outage. The specific changes are:

1) The time required for GPS to be lost or rejected before vehicles with airspeed sensors either reset to GPS or invoke the zero side-slip assumption to constrain drift has been reduced from 15 to 10 seconds
2) A duplicate zeroing of the GPS position offset has been removed
3) If the vehicle is a non hovering vehicle (eg a plane) then the speed at which the GPS offset is pulled back to zero after a reset is increased from 1 to 3  m/s