Fix background sync dialog

[papjul]

• Fix crash when delay is shorter than 2 characters.
• Update text when minutes/hours/days is checked or sync disabled
• Actually update preferences when there is a change
• Actually cancel work when sync is disabled

Side note: If my understanding is correct, the work is a one time job that schedules another one at the end of the process. So if that process fails, the next one will not be scheduled. Is there a reason you made it that way instead of a periodic job?

[bernaferrari]

When I did this, I think periodic job had some issues. It is probably super fine to use it now. Thanks for contributing!

[papjul]

Well, Android doesn't guarantee it will execute exactly every 17 min if that's what you asked, because it will prioritize depending on how much taskwork is already running (which is a good thing IMO)

But I wonder if we really need to offer that much granularity? Wouldn't predefined options like 30 min, 1 h, 2 h, 6 h, 12 h, 24 h be enough?

[bernaferrari]

It was predefined before, but I think I had some issues with the library I was using. It was a slider from 15min to 7 days. If you want, I accept anything.