[native] Fix handleCurrentUserFID querying for current user's FID

Summary: This addresses ENG-9230. The cause was a simple error from D12569. Flow failed to catch it because it wasn't sure that currentUserID wasn't the string "identities".

Test Plan: I deployed a release build to my iOS device and confirmed that the Profile -> Farcaster account screen now showed a linked Farcaster account.

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13313