Improve the automatic updates checks

The builds for each device are released more or less at the same
time of the day, so check when the last build was released and
schedule the next check accordingly. Also, allow multiple automatic
updates check in the same day. There's no need to limit the number
of checks since they are not frequent and this can affect negatively
the effectiveness of the new algorithm. In addition to that, remove
any pending oneshot alarm in case a subsequent check succeeds and
log every new alarm.

Change-Id: I4668f2e342e51d3578992eec6d8c270065d9aa21