Correct and improve calendar events updating algorithm.

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.

app/src/main/java/com/tommasoberlose/anotherwidget/receivers/WeatherReceiver.kt

@@ -35,8 +35,6 @@ class WeatherReceiver : BroadcastReceiver() {
     companion object {
         private const val MINUTE = 60 * 1000L
         fun setUpdates(context: Context) {
-            removeUpdates(context)
-
             if (Preferences.showWeather) {
                 val interval = MINUTE * when (Preferences.weatherRefreshPeriod) {
                     0 -> 30