Honor locale time settings in lock screen.

The time display in the lock screen should honor the LC_TIME variable,
including the locale-specific settings for whether to display a 12-hour or
24-hour time.  Sending the time and date strings through gettext results in
a value which is specific to LC_MESSAGES, which can be different from
LC_TIME.  Use the %X value for a time, which is guaranteed to be appropriate
for the locale in question.

In my particular case, I use UTC for my system time even though I live in the United States, so a 12-hour time is not very useful to me. I set LC_TIME to POSIX and LC_MESSAGES (and all the rest of the locale variables) to en_US.UTF-8, so passing the locale strings through gettext breaks things for me. Assuming people set their locales correctly, everything should just work.