refactor(oonimkall): expose the session close call

[DecFox]

Checklist

• I have read the contribution guidelines
• reference issue for this pull request: probe-engine: cannot run measurements using psiphon proxy probe#2810
• if you changed anything related to how experiments work and you need to reflect these changes in the ooni/spec repository, please link to the related ooni/spec pull request:
• if you changed code inside an experiment, make sure you bump its version number

Description

This diff exposes the Session.Close call for the geoip session we initialize in the mobile app. This is required to close the circumvention tunnel before we use start another tunnel as part of the async tasks to run the experiments.