datetime: Use accessible-description instead of -name for Hour & Minute

In my testing, default orca didn't seem to read the "Hour" or "Minute" words when we used accessible-name but it does work with accessible-description.

This is the only place we used accessible-name in gnome-control-center but we use accessible-description in a few other places.

Other Info

More specifically, it now reads:

"37 selected spinbutton Minute"

which seems good enough to me. Maybe "Minute" would be read earlier in the phrase if accessible-name worked how I would have expected.