[lib] Show cached ENS names on first render

Summary:
Instead of waiting for ENSCache to resolve a Promise, this diff makes it so if an ENS name is cached, we show it immediately from useStringForUser on the first render.

Depends on D6418

Test Plan: I added a console.log of the result of useStringForUser, and then logged out and back in on the iOS simulator. After this diff, the first render of useStringForUser was correct when the result was cached, but before this diff the first render had the ETH address instead of the ENS name

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6419