service: Display updated WatchdogUSec from sd_notify

Suppose a service has WatchdogSec set to 2 seconds in its unit file. I
then start the service and WatchdogUSec is set correctly:

    % systemctl --user show psi-notify -p WatchdogUSec
    WatchdogUSec=2s

Now I call `sd_notify(0, "WATCHDOG_USEC=10000000")`. The new timer seems
to have taken effect, since I only send `WATCHDOG=1` every 4 seconds,
and systemd isn't triggering the watchdog handler. However, `systemctl
show` still shows WatchdogUSec as 2s:

    % systemctl --user show psi-notify -p WatchdogUSec
    WatchdogUSec=2s

This seems surprising, since this "original" watchdog timer isn't the
one taking effect any more. This patch makes it so that we instead
display the new watchdog timer after sd_notify(WATCHDOG_USEC):

    % systemctl --user show psi-notify -p WatchdogUSec
    WatchdogUSec=10s

Fixes #15726.