[lib] Fix robotext for CREATE_SUB_THREAD to handle sidebar

Summary: We use the string 'sidebar' instead of 'child thread' now. D573 handled CREATE_THREAD and CREATE_SIDEBAR, but not CREATE_SUB_THREAD.

Test Plan: Flow

Reviewers: palys-swm, KatPo

Subscribers: zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D598