Migrate to mmkv or similar from async storage

[gmaclennan]

Currently we use async storage for persisting data in Mapeo. The main data we persist are:

• User settings
• Draft observations whilst the user is editing, before save
• Navigation state
• Selected language

Async storage is async (it's in the name!) and it needs to be because it is quite slow. However this adds a lot of complexity to our app, because everywhere that we persist data we need to handle a loading state (we generally do this by not rendering the app until everything has loaded).

An alternative option is react-native-mmkv which is fast and synchronous. Realm would also work, and we may well use it for a future implementation of Mapeo Core, but it might be a bit of a sledgehammer solution for storing user settings (although the support for migration might be really helpful).

This change would allow us to remove a lot of code that uses usePersistedState.js to remove the need for handling a 'status' (e.g. loading) for the persisted state.