Also minor Kotlin cleanups

Yes I know this is bad practice but it's just for testing purposes

Rename OK button and add cancel button

Apparently it's a Unix timestamp in milliseconds and this took me way
too long to figure out myself

When changing backup frequency settings, schedule the next backup using
the new setting, but relative to the timestamp of the latest backup,
instead of immediately.

This ensures that backups are not created too frequently. A backup will
only be immediately created if the latest backup was too long ago,
relative to the new frequency setting, or if there is no latest backup.

- Replace debug toast with debug log statement
- Use MaterialAlertDialog instead of boring AlertDialog

Just because the plural has a single manifestation in English doesn't
mean we don't need to have plurals for it in other languages.