[native] rename native themes-types to theme-utils

Summary:
Now that we lifted all the theme types into lib, the native file types would be better named as a utils file

Part of https://linear.app/comm/issue/ENG-4956/lift-any-necessary-shared-types-selectors-functions-for-storing-the

Depends on D9528

Test Plan: flow and confirmed that updating the app theme on native still works as expected

Reviewers: atul, inka, rohan

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9529