[in_app_purchase] Federated Android implementation (#3841)

* Implement BillingClientWrapper and unit-tests

* Android specific implementation

* Moved Android specific methods into addition class

* Added missing line end

* Purchases status to restored after call restorePurchases

* Don't force GooglePlayPurchaseParam

* Implement registerPlatform method

* Added TODO comment to add example

* Fixed mistake in API documentation

* Added additional assert to test enablePendingPurchase

* Update Android implementation with latest platform_interface

Author: mvanbeusekom

packages/in_app_purchase/in_app_purchase_android/CHANGELOG.md

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+## 0.1.0
+
+* Initial open-source release.

packages/in_app_purchase/in_app_purchase_android/LICENSE

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+Copyright 2013 The Flutter Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+    * Neither the name of Google Inc. nor the names of its
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

packages/in_app_purchase/in_app_purchase_android/README.md

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+# in_app_purchase_android
+
+The Android implementation of [`in_app_purchase`][1].
+
+## Usage
+
+### Import the package
+
+This package has been endorsed, meaning that you only need to add `in_app_purchase`
+as a dependency in your `pubspec.yaml`. It will be automatically included in your app
+when you depend on `package:in_app_purchase`.
+
+This is what the above means to your `pubspec.yaml`:
+
+```yaml
+...
+dependencies:
+  ...
+  in_app_purchase: ^0.6.0
+  ...
+```
+
+If you wish to use the Android package only, you can add  `in_app_purchase_android` as a
+dependency:
+
+```yaml
+...
+dependencies:
+  ...
+  in_app_purchase_android: ^1.0.0
+  ...
+```
+
+## TODO
+- [ ] Add an example application demonstrating the use of the [in_app_purchase_android] package (see also issue [flutter/flutter#81695](https://github.com/flutter/flutter/issues/81695)).
+
+
+[1]: ../in_app_purchase/in_app_purchase

packages/in_app_purchase/in_app_purchase_android/analysis_options.yaml

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+include: ../../../analysis_options_legacy.yaml

packages/in_app_purchase/in_app_purchase_android/android/build.gradle

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+group 'io.flutter.plugins.inapppurchase'
+version '1.0-SNAPSHOT'
+
+buildscript {
+    repositories {
+        google()
+        jcenter()
+    }
+
+    dependencies {
+        classpath 'com.android.tools.build:gradle:3.3.0'
+    }
+}
+
+rootProject.allprojects {
+    repositories {
+        google()
+        jcenter()
+    }
+}
+
+apply plugin: 'com.android.library'
+
+android {
+    compileSdkVersion 29
+
+    defaultConfig {
+        minSdkVersion 16
+        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+    }
+    lintOptions {
+        disable 'InvalidPackage'
+    }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+}
+
+dependencies {
+    implementation 'androidx.annotation:annotation:1.0.0'
+    implementation 'com.android.billingclient:billing:3.0.2'
+    testImplementation 'junit:junit:4.12'
+    testImplementation 'org.json:json:20180813'
+    testImplementation 'org.mockito:mockito-core:3.6.0'
+    androidTestImplementation 'androidx.test:runner:1.1.1'
+    androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.1'
+}

packages/in_app_purchase/in_app_purchase_android/android/gradle.properties

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+org.gradle.jvmargs=-Xmx1536M

packages/in_app_purchase/in_app_purchase_android/android/gradle/wrapper/gradle-wrapper.properties

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+#Mon Oct 29 10:30:44 PDT 2018
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.10.1-all.zip

packages/in_app_purchase/in_app_purchase_android/android/settings.gradle

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+rootProject.name = 'in_app_purchase'

packages/in_app_purchase/in_app_purchase_android/android/src/main/AndroidManifest.xml

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+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+  package="io.flutter.plugins.inapppurchase">
+</manifest>

packages/in_app_purchase/in_app_purchase_android/android/src/main/java/io/flutter/plugins/inapppurchase/BillingClientFactory.java

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+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package io.flutter.plugins.inapppurchase;
+
+import android.content.Context;
+import androidx.annotation.NonNull;
+import com.android.billingclient.api.BillingClient;
+import io.flutter.plugin.common.MethodChannel;
+
+/** Responsible for creating a {@link BillingClient} object. */
+interface BillingClientFactory {
+
+  /**
+   * Creates and returns a {@link BillingClient}.
+   *
+   * @param context The context used to create the {@link BillingClient}.
+   * @param channel The method channel used to create the {@link BillingClient}.
+   * @param enablePendingPurchases Whether to enable pending purchases. Throws an exception if it is
+   *     false.
+   * @return The {@link BillingClient} object that is created.
+   */
+  BillingClient createBillingClient(
+      @NonNull Context context, @NonNull MethodChannel channel, boolean enablePendingPurchases);
+}