Save favorites so that widgets can read them

[geosaaga]

Widgets need access to favorites defined in the main app. Save favorites to UserDefaults (iOS) and shared preferences (Android) when favorites are modified and when the app is launched.

react-native-mmkv seems to be the best library to implement this

https://github.com/mrousavy/react-native-mmkv

iOS requires that the main app and widget belong to same app group to share settings.

[geosaaga]

Current widgets read favorites from SQLiteDatabase where react-native-async-storage saves data. Have to keep that working.

[geosaaga]

PR: #712

In the end didn't use react-native-mmkv, because that would have mean bigger changes => integrating mmkv library also to iOS/Android native code.