[Persisted] Make broadcast subscriptions granular by key (#4874)

* Add fast path for guaranteed noop updates

* Change persisted.onUpdate() API to take a key

* Implement granular broadcast listeners

src/state/preferences/languages.tsx

@@ -43,8 +43,8 @@ export function Provider({children}: React.PropsWithChildren<{}>) {
   )
 
   React.useEffect(() => {
-    return persisted.onUpdate(() => {
-      setState(persisted.get('languagePrefs'))
+    return persisted.onUpdate('languagePrefs', nextLanguagePrefs => {
+      setState(nextLanguagePrefs)
     })
   }, [setStateWrapped])