1. UpdateCalendarService will fetch all events from now to the next fetch time + Preferences.showUntil, so that EventRepository holds all events need to be shown before the next fetch;
2. Every event is updated 1 minute after the corresponding time span, to make the actual time difference consistent with the displayed getRelativeTimeSpanString;
3. Update events only when necessary, eliminate redundant alarms; cancel all update alarms when showEvents is turned off.
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