New M3 Demo App for Master Channel (#1378)

* Added components

* Added test

* Added README

* Run flutter format

* Added all the updated components and tests

* Added README file

* Update README.md

* Add textfields to M3 demo app

* move textfield to exprerimental/

* Update component_screen_test.dart

* Fixed comment: changed sdk version

* Changed all double quotes to single

* Added trailing comma, changed all private fields to public except private state classes

* Added enum for screen selection

* Removed comment for material_3_demo

* Added experimental m3 demo app to master CI pipeline

* fixed failing tests

* Modified the analysis_options.yaml

* Fixed warning

* Ran flutter format command

Co-authored-by: Qun Cheng <quncheng@google.com>
Co-authored-by: hangyu <108393416+hangyujin@users.noreply.github.com>

experimental/material_3_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"