store: use @react-native-community/async-storage

Switch to actually using @react-native-community/async-storage.

The module is written in Flow under the hood, but strangely doesn't
expose the Flow types in its standard entry point. We therefore import
directly from the relevant interior file to get proper type
definitions.

This will have to be revisited in the future, as the module is
currently being completely rewritten to use TypeScript.

Fixes zulip#3570.

Author: rk-for-zulip

src/boot/ZulipAsyncStorage.js

@@ -1,5 +1,6 @@
 /* @flow strict-local */
-import { AsyncStorage, NativeModules } from 'react-native';
+import AsyncStorage from '@react-native-community/async-storage/lib/AsyncStorage';
+import { NativeModules } from 'react-native';
 import * as logging from '../utils/logging';
 
 export default class ZulipAsyncStorage {