Polymorphic Serialization of ScreenParams.

[luca992]

This builds upon my other pr #25.

It switches to using polymorphic serialization of ScreenParams with kotlinx.serialization.

Note: I moved all ScreenParams implementations in eu.baroncelli.dkmpsample.shared.viewmodel.screens.ScreenParams instead of inside a screen's eu.baroncelli.dkmpsample.shared.viewmodel.screens.screen-name subpackage, because all classes implementing a sealed interface must be in the same package. The reason I used a sealed interface is that it allows the list of ScreenParams subclasses that can be serialized in a polymorphic way to be determined at compile time vs having to explicitly registered them at runtime.

• See: https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism

Also this fixes the issue where the app crashes on start if you change variable names inside ScreenParams after relaunching the app. Previously this causes a deserialization error when reading the serialized ScreenParams from local settings. When a deserialization error is encountered this pr will work around the issue by retrying with the default params for screen.

[luca992]

I'll recreate this pr this after you look at #28