Use shared ViewModel in Fragments

[petoalbert]

The WeatherApp sample does not work correctly, because the Fragment uses a different ViewModel than the Activity. It seems like it was forgotten to be updated after 0.9.1. Since the sampleapp-arch was also created before updating to 0.9.1, I guess it was also meant to be used with a shared ViewModel.

[dwkim-sugarhill]

👍 we need this PR