fixed Gradle setup

[SaeedZhiany]

• Loaded 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 buildToolsVersion, because it's no longer needed to specify
  newer versions of the Android Gradle Plugin itself chooses the correct version of BuildTools automatically

[Quantum-35]

Good catch @SaeedZhiany.
Can you also add this comment to ReadMe before installation?
` - If you are using React Native version 60, you will need to change the platform version in react-native-biometrics.podspec to version 9.0 so that you can successfully run pod install without version errors.

s.platform = :ios should be same as node_modules/react-native/third-party-podspecs/Folly.podspec
`

[SaeedZhiany]

@Quantum-35

My change is not related to iOS, why it should be added in this PR?!
The change is only related to android builds and even users don't need to be informed about it because it's not a break-change (it just prevents from downloading unwanted gradle stuffs on building times)

[SaeedZhiany]

@NappyPirate, can you please review and merge this?

[SaeedZhiany]

@NappyPirate, can you please review and merge this?

@NappyPirate we are still waiting... please review and merge this PR. Thanks