Added M3 demo app (#1189)

* Added M3 demo app

* Changed to use the centralized lint configuration.

* Moved the app at the root level

* Added to CI

* Added app to beta CI and commented app on stable CI file

* Update README.md

* Addressed comments

* fixed comments

* Reverted app name in CI files.

* Added project to the Pub Dependabot config

* Linting pass for current beta channel standards

Co-authored-by: Qun Cheng <quncheng@google.com>
Co-authored-by: Craig Labenz <craig.labenz@gmail.com>

material_3_demo/android/build.gradle

@@ -0,0 +1,31 @@
+buildscript {
+    ext.kotlin_version = '1.6.10'
+    repositories {
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        classpath 'com.android.tools.build:gradle:4.1.0'
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+    }
+}
+
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+rootProject.buildDir = '../build'
+subprojects {
+    project.buildDir = "${rootProject.buildDir}/${project.name}"
+}
+subprojects {
+    project.evaluationDependsOn(':app')
+}
+
+task clean(type: Delete) {
+    delete rootProject.buildDir
+}