GUI: Use standard routine for auto-reboot

If 'reboot after install' is enabled, follow the standard reboot
routine to process 'No OS' and 'Install SuperSU' checks.

Since the system reboot routine is repeated several times, it has been
separated into its own routine.

Change-Id: I4c1882ecb8dc2d153ec2378b2e0973ab1a2a8278