chore: add flutter quickstart example, assets and update README

examples/flutter/quickstart/android/build.gradle

@@ -0,0 +1,18 @@
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+rootProject.buildDir = "../build"
+subprojects {
+    project.buildDir = "${rootProject.buildDir}/${project.name}"
+}
+subprojects {
+    project.evaluationDependsOn(":app")
+}
+
+tasks.register("clean", Delete) {
+    delete rootProject.buildDir
+}