Adding state hoisting in order to have the compose preview available.

[jgavazzisp]

Hi there, thanks for creating the project to nudge the community towards the recommended way to work with Firebase.

I just had one suggestion for now which is to do a state hoisting on the screen — I've submitted one example and can do the same for the other ones if you guys are happy with it —.

After doing the state hoisting we can add the preview tag to the screen and enable live edits — when using Android Studio Electric Eel — to have fast feedback on UI changes.

Please feel free to discard the PR if you feel it is not useful.

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