Add support of namespace property to support Android Gradle Plugin 8

[Skyost]

This PR adds support of namespace property to support Android Gradle Plugin (AGP) 8. It fixes the following error :

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring project ':game_services'.
> Could not create an instance of type com.android.build.api.variant.impl.LibraryVariantBuilderImpl.
   > Namespace not specified. Please specify a namespace in the module's build.gradle file like so:

     android {
         namespace 'com.example.namespace'
     }

     If the package attribute is specified in the source AndroidManifest.xml, it can be migrated automatically to the namespace value in the build.gradle file using the AGP Upgrade Assistant; please refer to https://developer.android.com/studio/build/agp-upgrade-assistant for more information.

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 1s
Running Gradle task 'bundleRelease'...                           2 150ms
Gradle task bundleRelease failed with exit code 1

See flutter/flutter#125181 (comment).

[Skyost]

If you want to try this, in your pubspec.yaml :

dependencies:
  games_services:
    git:
      url: https://github.com/Skyost/games_services.git
      ref: fix/agp-8
      path: games_services

[Abedalkareem]

Amazing! Thanks!