[lib] Use setLocalFID when updating local FID to match identity

Summary:
unlinkFID was wrong here because:

1. We don't need to contact identity to update an FID we got from identity
2. If the FID is non-null, we end up erasing it

Instead we should just update the local FID to match identity.

Depends on D13737

Test Plan: Careful analysis of this case (it happens when identity FID doesn't match local FID)

Reviewers: will

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13738