restructured the add to app samples (#698)

add_to_app/books/android_books/app/build.gradle

@@ -0,0 +1,49 @@
+apply plugin: 'com.android.application'
+apply plugin: 'kotlin-android'
+apply plugin: 'kotlin-android-extensions'
+
+android {
+    compileSdkVersion 29
+    buildToolsVersion "29.0.3"
+
+    defaultConfig {
+        applicationId "dev.flutter.example.books"
+        minSdkVersion 16
+        targetSdkVersion 29
+        versionCode 1
+        versionName "1.0"
+
+        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+    }
+
+    buildTypes {
+        release {
+            minifyEnabled false
+            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
+        }
+    }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+    kotlinOptions {
+        jvmTarget = "1.8"
+    }
+}
+
+dependencies {
+    implementation fileTree(dir: "libs", include: ["*.jar"])
+    implementation "com.squareup.okhttp3:okhttp:4.7.2"
+    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
+    implementation 'androidx.core:core-ktx:1.3.0'
+    implementation 'androidx.appcompat:appcompat:1.1.0'
+    implementation "androidx.activity:activity-ktx:1.1.0"
+    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
+    implementation 'com.google.android.material:material:1.1.0'
+    implementation 'com.google.code.gson:gson:2.8.6'
+    implementation project(path: ':flutter')
+    testImplementation 'junit:junit:4.13'
+    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
+    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
+
+}