[lib] Change generatePendingThreadColor to accept only an array

Test Plan: Login on two devices as different users. Search for a pending thread with both users and one another. Check if thread color is the same on both devices.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: ashoat, KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1033