plugins/nightlight: Prevent preview temperature reset and fix preview timer callback
1st commit: After reaching the target preview temperature, skip resetting the slow update timers to avoid reverting the temperature to the normal value. This is the actual bug described below.
2nd commit: When the timer times out, it is not active anymore. Instead only check if it exists and delete it afterwards. Otherwise the preview never ends.
I'm open to better wording if you have suggestions.