send is_fresh_instance for subscribe events

Summary: `subscribe` events should receive `is_fresh_instance` notifications.

Test Plan: `arc unit --everything`

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D916732

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extra : rebase_source : b564095fc72b246a177b5a243a877294df51c876
extra : amend_source : 9f3a838e3f011779f7c52b01b23815526592913d