[web] Modify selectedThreadColors to preserve order of keys

Summary:
Addresses the issue flagged by @palys-swm here: https://phabricator.ashoat.com/D3702?id=11317#inline-22176

I tried a few other ways to do this, but ran into various flow issues. This aproach works, but it seems overly complex. Will continue investigating alternative approaches.

Test Plan: flow, colors now appear in the correct order:

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: Adrian, karol-bisztyga, benschac, yayabosh, palys-swm

Differential Revision: https://phabricator.ashoat.com/D3715