Add support for Unity with Gradle 8+

[manugildev]

Starting with AGP 8+, module's build.gradle files are required to have the namespace property. This is supposed to replace the package attribute on the AndroidManifest.xml file.

Error before:

A problem occurred configuring project ':unityLibrary:mobilenotifications.androidlib'.
> Could not create an instance of type com.android.build.api.variant.impl.LibraryVariantBuilderImpl.
   > Namespace not specified. Specify a namespace in the module's build file. See https://d.android.com/r/tools/upgrade-assistant/set-namespace for information about setting the namespace.

Manual testing:

• ✅ Tested using the three TestProjects included in the package.
• ✅ All the AutomatedTests are passing locally as well.
• ✅ Tested 2021.3, 2022.3 and 2023.3 (trunk). No issues found.

Automated tests:

• ✅ Automated tests passing for all supported Unity versions.