[lib] Invalidate UserIdentityCache when changing FID

Summary:
When we change the current user's FID, we should invalidate their entry in the UserIdentityCache. Otherwise, FarcasterDataHandler will continue to use the old value, and can end up wiping the FID if handleCurrentUserFID is run again before the app is killed (this can happen if the app is backgrounded/foregrounded).

This addresses ENG-9704.

Depends on D13736

Test Plan:

1. Open mobile app when Farcaster account not linked
2. Link Farcaster account
3. Background the app
4. Foreground the app
5. Confirm that the Farcaster account is not unlinked

Reviewers: will

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13737