[Digital Clock] Fix 24h tri-state button broken in port to QQC2

In 12h system locales (eg: en_US), checked=1 is cast as
Qt::PartiallyChecked=1 which defaults to the system locale (12h).
Since it's no longer a tri-state button, the user can't select
Qt::Checked=2 which forces the 24h state.

BUG: 404292

FIXED-IN: 5.15.1