loaded Android Gradle Plugin Conditionally

[SaeedZhiany]

This wraps the Android Gradle plugin dependency in the buildscripts section of android/build.gradle in a conditional:

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

The Android Gradle plugin is only required when opening the project stand-alone, not when it is included as a dependency. 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).

for more info, you can refer to this thread and especially this comment.

[wn-na]

Thanks for the great way to do it :)
It's nice to get rid of an unnecessary step.

I'll include this in the next 1.8.0 release. LGTM @SaeedZhiany