refreshing core on uc16 leads to conflicting duplicated reboot notices

[Ian Johnson]

Running snap refresh core on a UC16 device I initially see the first reboot notification for 10 minutes from now:

```
Broadcast message from root@device (Thu 2019-07-11 14:36:48 UTC):

reboot scheduled to update the system
The system is going down for reboot at Thu 2019-07-11 14:46:48 UTC!

2019-07-11T14:36:48Z INFO Waiting for restart...
snapd is about to reboot the system
```

Then almost immediately thereafter I see another message for only 1 minute from now:

```
Broadcast message from root@device (Thu 2019-07-11 14:36:51 UTC):

reboot scheduled to update the system
The system is going down for reboot at Thu 2019-07-11 14:37:51 UTC!
```

Ideally, we would only have one notification show up with the shorter of the times for a single snap refresh.

[Ian Johnson (anonymouse67)]

I actually see this on Ubuntu Core 18 as well

[OSP Snap Administrator Development (ospdev-snap-administrator)]

I'm seeing this on UC20 too.

# snap reboot --run

Broadcast message from <email address hidden> (Thu 2021-03-11 17:35:34 UTC):

reboot scheduled to update the system
The system is going down for reboot at Thu 2021-03-11 17:45:34 UTC!

Reboot into "run" mode.