update main activity
update main activity

 - use config/utils
 - add conditional GCM registration/fallback
iAMr00t/ota-update-centre,SXDXMZ2/system_app_SXD-Update-Center,OTAUpdateCenter/ota-update-centre,iAMr00t/ota-update-centre,pcarenza/packages_apps_ota_update_centre