Fix Unity 6 Android compatibility with gradle

[jkasten2]

Confirmed this change works with Unity 6.0.42f1 and is still compatible with the Unity's oldest LTS, 2021.3.45f1.

Description

One Line Summary

Fixes Android build issue with Unity 6 as it's Gradle version requires namespace to be defined in the build.gradle.

Details

This addresses Issues:

• [Bug]: Unity 6 Preview Gradle Build Failed #752
• [Bug]: Incorrect OneSignalConfig.androidlib/build.gradle Fails to make Android build on Unity 6 #775

Motivation

SDK should continue to be compatible with newer versions of Unity, without work arounds.

Scope

Only effects the Android build process that runs on app developers projects.

Testing

Unit testing

No testing suite that builds an app end-to-end in place.

Manual testing

Testing a new Unity project and ensure an Android APK was built successfully on:

• Unity 6.0.42f1
• Unity 2021.3.45f1 (Unity's oldest LTS)

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes
• Android build system

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

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This change is