unlockDialog: Also center the time in 12h format

unlockDialog: Also center the time in 12h format

Since the time string uses a space to pad single-digit numbers, the
centered label looks off-center. Fix this by applying the trim()
function to get rid of whitespaces.

The drawback of this approach is that the label will change position
slightly when the time changes, but for the lockscreen that's okay.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6193
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3290>