New location for "googlePlayServicesVersion" in NativeScript 5+

[tiagoblackcode]

In NativeScript 5+, the Gradle property that allows the SDK version to be customised must now be placed in app/App_Resources/Android/before-plugins.gradle instead of app/App_Resources/Android/app.gradle.

The change in question is NativeScript/android#1185 released on version 5 (https://github.com/NativeScript/android-runtime/blob/master/CHANGELOG.md#500).

Placing it in app/App_Resources/Android/app.gradle won't work.
The documentation should be updated to reflect this.