[Adaptive JetStream] light and dark color schemes are exactly the same

[dturner]

The color schemes used in the app are exactly the same for both light and dark modes:

https://github.com/android/adaptive-apps-samples/blob/main/AdaptiveJetStream/jetstream/src/main/java/com/google/jetstream/presentation/theme/Color.kt

Either dark mode should be supported or it shouldn't and the dark color scheme should be removed.