restructured the add to app samples (#698)

add_to_app/plugin/android_using_plugin/app/build.gradle

@@ -0,0 +1,37 @@
+apply plugin: 'com.android.application'
+
+apply plugin: 'kotlin-android'
+
+apply plugin: 'kotlin-android-extensions'
+
+android {
+    compileSdkVersion 28
+    defaultConfig {
+        applicationId "dev.flutter.example.androidusingplugin"
+        minSdkVersion 19
+        targetSdkVersion 28
+        versionCode 1
+        versionName "1.0"
+        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+    }
+    buildTypes {
+        release {
+            minifyEnabled false
+        }
+    }
+    compileOptions {
+        sourceCompatibility 1.8
+        targetCompatibility 1.8
+    }
+}
+
+dependencies {
+    implementation project(':flutter')
+    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
+    implementation 'androidx.appcompat:appcompat:1.1.0'
+    implementation 'androidx.core:core-ktx:1.1.0'
+    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
+    testImplementation 'junit:junit:4.12'
+    androidTestImplementation 'androidx.test.ext:junit:1.1.0'
+    androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.1'
+}