update monitor: Fix autoupdates triggering in a feedback loop

Only trigger autoupdates once a day, and avoid accidentally triggering them when a plugin emits updates-changed. Otherwise it can unexpectedly happen that a user installs an app, which triggers updates-changed, which in turn triggers autoupdates immediately.

Worse, a plugin might emit updates-changed when it ran into a failure, which causes autoupdates to trigger, which in turn runs into another failure, which causes updates-changed, and so on.

Instead, make sure autoupdates only get triggered once per day.