Add option to sync every 30 minutes

[0xedward]

I wanted more granularity between the current existing sync intervals of "Every 15 minutes" and "Every hour". I thought others may want this interval as well.

Here's a few screenshots showing the interval is properly set and retrieved from android.content.ContentResolver (note: period and seconds are both set to 1800 seconds when Every 30 minutes is selected:

I didn't update the strings.xml files for other languages because I didn't want to mess up the translation.

[tasn]

Could you please fix all of the translations? Because of how the translation arrays work, they'll all have issues unless you add a 30min variant. I think it's safe enough to just copy the 15min variant in each and change the 15 to 30.

[0xedward]

@tasn I added the 30 minute interval in the other strings.xml files 🙂

[tasn]

Thanks a lot!