[lib] Make sure ThreadInfo.uiName only includes the first line

Summary: This is a follow-up to D861 – more context there.

Test Plan: Flow

Reviewers: atul, karol-bisztyga, palys-swm

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D864