The statistics module settings (admin/config/system/statistics) need to be converted to use the new configuration system.
- Create a default config file and add it to the statistics module.
- Convert the admin UI in statistics.admin.inc to read to/write from the appropriate config.
- Convert any code that currently accesses the variables used to use the config system.
- Write an upgrade path from D7 -> D8 to migrate existing data to the new system and drop the appropriate variables.
Patch attached that covers the first 3 points. Waiting for #1497132: Provide a generalised function to update variables from Drupal 7 to Drupal 8's configuration management system to cover the upgrade.