[lib][native][web] Make Flow types for Redux state read-only
Summary:
This probably should've been done a long time ago. Best practice is to modernize a Flow type whenever you touch it, but evidently people were too intimidated to try and modernize BaseAppState and its various related types.
I went ahead and did this. It looks like it went well, with no new Flow errors.
Test Plan: Flow
Reviewers: inka, michal, patryk
Reviewed By: inka
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D8685