Updated naming convention for the tread creation in account creator

Summary: Updated the naming convention of the thread creation from personalThreadResults to privateThreadResults

Test Plan: Created a new account and made sure the private threads still worked. Tested with old accounts too

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D643