[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