[native] Add messageStoreLoaded to SQLiteContext

Summary:
We use SQLiteContext to determine whether the state persisted in SQLite has been loaded.

For example, in app-navigator, we won't render within App.Navigator until storeLoadedFromLocalDatabase is true.

Depends on D4158

Test Plan:
We're manually setting messageStoreLoaded to true in SQLiteContextProvider for persistConfig.version < 31 (the migration which "flips the switch") for now.

The app should continue to work as expected without issue. Specifically, will deploy a release build to my phone and open/force quit/open/etc the app a couple times and make sure things continue to work as expected.

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4159