the user didn't decline the update, he canceled the download. This might be because he's going out of a Wi-Fi zone. 

The fact that we don't remove the notification in this case makes this clear to me here. To be consistent we should still have it at reboot.

Anyway, we already decided that the user will be bugged about the system update (remember it's not possible to uncheck the system update in the update prompt). We can ask for a confirmation from the UX guys but I'm quite sure of their answer here.