Improve background update check

[SommerEngineering]

The background update check currently checks at fixed intervals. If a user puts their device into sleep mode, e.g. for 12 hours, the interval then continues. With a 1-hour update check, however, users expect the update check to take place immediately after 12 hours of sleep.

We should therefore implement the update check on a calendar basis. We use the calendar and time to determine the next update check time. The actual update check then takes place at 3-minute intervals, for example. This means that the update check is carried out within a maximum of 3 minutes after the device wakes up.

See also #26