Prevent going to online after boot when SIM inserted and Offline enabled

Currently ril driver was forcing modem online always in boot when SIM was present. This was also causing UI and modem being in out of sync regarding online status since UI was not notified in this case. Setting modem online/offline is in the responsibility of UI/client, so removed the power_on method since not needed anymore (was needed earlier when set_online support was not yet implemented).