prev_mail_system is not saved after being set

Problem/Motivation

prev_mail_system is not saved after being set.

Steps to reproduce

1. Set some value for system.mail interface.default: drush cset system.mail interface.default foo
2. Enable the SMTP module
3. Visit /admin/config/system/smtp
4. Select "On" for "Set SMTP as the default mailsystem"
5. Click "Save configuration"
6. Run drush cget smtp.settings and observe that prev_mail_system is not set to foo

Proposed resolution

Save prev_mail_system after setting it.

Remaining tasks

User interface changes

N/A

API changes

N/A

Data model changes

N/A