fix(gradle): Load Android Gradle Plugin conditionally (#1230 by @SaeedZhiany)
BREAKING CHANGE:
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).
Also moved getExtOrDefault to buildScript block to able to use everywhere in the file
This change shouldn't break any apps, but we are marking it as a breaking change in case there are some use cases we've missed.
[skip ci]