[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

lib/components/farcaster-data-handler.react.js

@@ -172,7 +172,7 @@ function FarcasterDataHandler(props: Props): React.Node {
       : getCachedUserIdentity(currentUserID) === undefined;
 
     if (fid && !identityFIDRequestTimedOut && fid !== identityFID) {
-      await unlinkFID();
+      setLocalFID(identityFID);
       return;
     } else if (fid) {
       const isCurrentUserFIDValid =