improve correctness of GNSS datetime handling

[edgecase14]

This prevents initial positioning window not being greyed out even when there has never been a GNSS fix.

This also avoids pretending the system time is the GNSS time in EXIF metadata. That could potentially break someone's batch image processing, so maybe add to release notes.

[edgecase14]

Additional comments in issue #6571

[gdt]

I'm an outsider, but were I reviewing for merge i'd want to see the multiple commits rebased into tthe series of commits you would have done had you done it perfectly the first time. Not a criticism of getting to working incrementlaly, , but On projects I maintain I prefer to merge clean commits, vs squashing things that don't necessarily go together. I don't know how qfield feels.