[lib] rename parentThreadInfo -> threadInfo in getCommunity

Summary:
Depends on D13753. Follow up from D13745

old param name suggested that the function would only work for child threads in a community but the function actually returns the community for the community root thread as well

Test Plan: flow

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13769