[lib] Narrow threadInfo argument of threadFrozenDueToViewerBlock

Summary:
This change is kind of useless. Thought I'd be able to do a lot more narrowing of types "along" the memberHasAdminPowers callstack, but this was pretty much it. Figured I may as well put it up.

Depends on D12265

Test Plan: NA, flow

Reviewers: ashoat, ginsu, tomek, varun

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D12266