[identity] Ask Tunnelbroker to terminate old device sessions

Summary:
Address ENG-8218.

Old Tunnelbroker sessions should be terminated when primary backup is restored (to avoid having both old and new primary device connected at the same time).

Depends on D13226, D13228

Test Plan: Repeated plan from D13228, now by spawning the Tunnelbroker task in Identity

Reviewers: kamil, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13229