Fix build.gradle setup

[friederbluemle]

Summary

Opening the android folder in Android Studio (or attempting to build it on command line) results in the following error:

ERROR: Plugin with id 'com.android.library' not found.

The reason was a missing buildscripts section in android/build.gradle. However, since the Android Gradle plugin is only required when opening the project stand-alone, the dependency was wrapped in a conditional:

if (project == rootProject) {
    // ... (dependency here)
}

By doing this, the project opens correctly in Android Studio, and it can also be consumed as a native module dependency from an application project without affecting the app project (avoiding unnecessary downloads/conflicts/etc).

A couple of other dependency fixes were required as well: com.facebook.react:react-native:+ is supposed to be included and resolved from a local path in node_modules. Therefore it needs to be added to the devDependencies section of the project. Without it, Gradle would pick the latest version of React Native from Maven Central: 0.20.1 from Feb 2016 😱.

Test Plan

What's required for testing (prerequisites)?

Android Studio and/or Gradle installed locally.

What are the steps to reproduce (after prerequisites)?

Open android in Android Studio:

$ studio android/

Or:

Generate a Gradle wrapper on the fly, and build on command line:

cd android/
gradle wrapper --gradle-version 5.4.1 --distribution-type all
./gradlew build

Successfully verified and tested on:

• Android Device (Xiaomi Mi A2)
• Android Emulator (Pixel 3a)
• iOS Device (iPhone 8)
• iOS Simulator (iPhone X)

Compatibility

OS	Implemented
iOS	✅
Android	✅

Checklist

• I have tested this on a device and a simulator
• I added the documentation in README.md (N/A)
• I mentioned this change in CHANGELOG.md (N/A)
• I updated the typed files (TS and Flow) (N/A)
• I added a sample use of the API in the example project (example/App.js) (N/A)

[zoontek]

@friederbluemle Thanks, I didn't know about this 👌🏼