[CommCoreModule][native_rust_library] move constants to one place

Summary:
Part of ENG-9656

I'm not sure why it was previously defined in two places. It makes it hard to maintain, so I'm cleaning this up. I added the missing comment.

Depends on D13937

Test Plan: Tested creating backup and secondary device login

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14000