[lib] Simplify getRelativeMemberInfos

Summary:
In subsequent diff going to split into two separate functions: getLegacyRelativeMemberInfos and getMinimallyEncodedRelativeMemberInfos and then toggle between then based on whether input RawThreadInfo is minimallyEncoded or not.

Since that diff is about duplicating existing functionality and branching... figure changing the actual functionality may be annoying for reviewers even though it's minimal (?)

Depends on D10227

Test Plan: CI/flow/etc.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10233