[lib] Fix robotext for CREATE_SUB_THREAD to handle sidebar
[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

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