Stopping the FeedSyncService after the sync is done.
Automated syncs every 12 hours.
Sync on background won't happen if the data is fresh enough.

Created global Config class for centralized access to shared preferences data.
Minor refactoring of FeedSyncService.
Feed sync at the start won't happen if the data is fresh enough.
Added Preferences screen with one checkbox (sync only on Wifi).
Checking of the "background sync only on Wifi" flag and also current connection when doing sync.