AP_NavEKF: Improve robustness to long periods without GPS

This fixes a vulnerability to long periods without GPS (in excess of a minute) that in SITL has been observed to cause a loss of attitude reference. This has been fixed by:

1) Increasing the default accelerometer process noise for planes from 0.25 to 0.5 m/s^2 (this also improves robustness to aliasing effects). This takes it back to the original value before it was reduced to match the value that had worked well for copter, Experience with plane logs has shown they have larger accelerometer errors than copters due to higher g's and vibration levels as the plane user base is less educated in terms of isolating boards from vibration than the copter user base.
2) Reducing the default GPS timeout when using an airspeed sensor (time without GPS required to declare position and velocity bad) from 20 to 15 seconds.
3) Fixing a bug introduced with the introduction of optical flow that prevented lack of GPS data starting the GPS timeout which also prevented the zero side-slip fusion assumption being activated following loss of GPS data.

The plane in SITL can now fly for several minutes without GPS provided it has an airspeed sensor.