Provide default PreferenceDataStore implementation

[pakoito]

In Android O, individual preferences or even the entire PreferenceManager can call setPreferenceDataStore(), allowing your app to keep the same simple key/value pair API, but provide your own mechanism for storing the underlying data.

The interface is quite simple, it shouldn't take long to write. My time is limited right now, and if yours is too we'll have to wait until either of us is freed :D

https://developer.android.com/reference/android/preference/PreferenceDataStore.html

[pilgr]

Out of scope. PreferenceDataStore provides API to save only primitive types, this is not the real usage case for Paper. Developers choose Paper because of how easy it allowed to save complex objects. So I'm closing this ticket.