[native] Refactor app-navigator.js

Summary:
tab navigator and overlay navigator should be in separate files.
Linear issue: https://linear.app/comm/issue/ENG-1883/refactor-app-navigatorjs-so-each-navigator-has-its-own-file

Test Plan: run ios simulator

Reviewers: tomek, atul, ginsu, bartek

Reviewed By: tomek, ginsu, bartek

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5533