Restore Google Authenticator accounts from ADB backup
You cannot get the recovery keys (which are used to create backup codes) from adb backup of Google Authenticator because they do not exist in that backup. Adb backup of Google Authenticator only backup the user preferences of that app. 

Furthermore, if you took a single adb backup, hoping all apps would have been backed up that way, than I'm afraid even the user preferences were not backed up. A specific flag is used for Authenticator when backing up through adb. Without that, the app cannot be backed up. You can read more on that in my answer here.