Refactor ReadingListFragment to use ViewModel.

[terofeev]

What does this do?

• Refactored ReadingListFragment to use ViewModel.
• Added ReadingListFragmentViewModel and moved the update list logic into it.

Why is this needed?

Tech-debt.
Reduce complexity by simplifying the UI layer.

Phabricator:
https://phabricator.wikimedia.org/T303935

[terofeev]

Added wrapper class because the data has non-nullable type T.

open class Resource<T> {
    ...
    class Success<T>(val data: T) : Resource<T>()
    ...
}