Improved Gradle setup

[SaeedZhiany]

• Load Android Gradle Plugin conditionally

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.

• removed unnecessary buildToolsVersion specification

in newer versions of Android Gradle Plugin, it's no longer needed to specify buildToolsVersion the plugin itself will peek the correct version automatically.

• extracted duplicated default versions extraction from the root project Gradle setup into a function

it's a common between RN libraries to use auxiliary function safeExtGet to get rid the duplicated codes for extracting versions from the root project'S Gradle setup

[SaeedZhiany]

@faizalshap

please review and merge this.

Thanks