restructured the add to app samples (#698)

add_to_app/plugin/android_using_plugin/build.gradle

@@ -0,0 +1,26 @@
+// Top-level build file where you can add configuration options common to all sub-projects/modules.
+
+buildscript {
+    ext.kotlin_version = '1.3.50'
+    repositories {
+        google()
+        jcenter()
+    }
+    dependencies {
+        classpath 'com.android.tools.build:gradle:3.5.1'
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+        // NOTE: Do not place your application dependencies here; they belong
+        // in the individual module build.gradle files
+    }
+}
+
+allprojects {
+    repositories {
+        google()
+        jcenter()
+    }
+}
+
+task clean(type: Delete) {
+    delete rootProject.buildDir
+}