This changes the flow of the update triggering so that any Intent sent to
UpdateService can potentially trigger an update, depending only on the
state of the internet and the "Only on WiFi" preference. Instead of having
a timer that checks every hour to see if it is time to run the update, just
let AlarmManager send a trigger Intent based on the timing in the
"Update Interval" setting.
The update schedule is reset each time F-Droid restarts, and also each time
the user returns from the settings, so if AlarmManager fails us in the time
being, the updates will be rescheduled next time F-Droid is restarted, the
device is rebooted, etc.
refs #662
7d9f5e880c