[native] Generate pending thread color based on other user id

Summary: Currently changing search text resulted in generating new colors. To fix that, we can generate a color based on other user id.

Test Plan: Search for a pending thread. Change search text and verify that thread color remains the same.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D485