AP_GPS: don't consider uBlox PVT time correct unless we have 2D fix

[peterbarker]

the time may be set by an offline assistance client and may not be accurate

... this is not my commit, but it's in a WIP branch...

MAVProxy can supply this assistance data, so perhaps this patch is worth looking at?

[peterbarker]

Author: Andrew Tridgell <[email protected]>
Date:   Thu Jun 1 18:12:53 2017 +1000