Fix for the missing "Offline Ready" prompt

This fix makes sure that the "Offline ready, switch now?" prompt _will_ be displayed if the user quits the app while the offline sync is running.

I just added a simple `offline_mode_prompt` key to the default `SharedPreferences`, and made the main `OnlineActivity` check for its presence at startup.

It seems to work on my device (Galaxy Nexus, CM 10.1-RC5).

See: http://tt-rss.org/forum/viewtopic.php?f=1&t=2251