[keyserver/lib/native/web] introduce color-utils and move color functions from thread-utils to color-utils

Summary:
When working on D7178 I was getting the following error in keyserver:

D7178 should offer more context, but the TLDR:

We use selectedThreadColor to build a default user emoji avatar, and we are trying to access selectedThreadColor before it gets initialized when we build this user avatar deeper in thread-utils

Test Plan: flow and the error messages in the keyserver logs no longer appear.

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7177