[desktop] Type bridge object between desktop and web

Summary:
ENG-2219
Previously this bridge object (through which we send messages between electron and web app) was typed only on web. This diff moves the types to lib and uses them on desktop to keep everything in sync.

Test Plan: Run flow in desktop and web.

Reviewers: tomek, ginsu, inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6175