[native] [refactor] [ENG-530] organize constants and utils in chat directory

Summary: after @atul 's comment here: https://phabricator.ashoat.com/D3972, I organized the constants in the native directory and moved a function to the chat util files.

Test Plan: no new functionality. Everything works the same, no flow or eslint errors.

Reviewers: atul, palys-swm, dereknelson, def-au1t

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh, atul

Differential Revision: https://phabricator.ashoat.com/D3996