Next Gen UI demo (#1778)

First pass at a Next Generation UI demo app. The UI needs work, feedback
gratefully accepted.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

next_gen_ui_demo/android/settings.gradle

@@ -0,0 +1,11 @@
+include ':app'
+
+def localPropertiesFile = new File(rootProject.projectDir, "local.properties")
+def properties = new Properties()
+
+assert localPropertiesFile.exists()
+localPropertiesFile.withReader("UTF-8") { reader -> properties.load(reader) }
+
+def flutterSdkPath = properties.getProperty("flutter.sdk")
+assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
+apply from: "$flutterSdkPath/packages/flutter_tools/gradle/app_plugin_loader.gradle"