[user model] Added the namespace property in build.gradle

[nan-li]

Description

One Line Summary

This PR provide the package namespace if the AGP is equal or higher then 8.

Details

This PR is a cherry-pick of this main branch PR https://github.com/OneSignal/OneSignal-Flutter-SDK/pull/765 for the user_model branch.

Motivation

By adding the namespace condition for AGP 8 and higher, this fix the issue [Bug]: AGP 8 not supported #764

Testing

Manual testing

Tested during the Gradle sync with AGP 9

Affected code checklist

• [ ] Notifications
  • [ ] Display
  • [ ] Open
  • [ ] Push Processing
  • [ ] Confirm Deliveries
• [ ] Outcomes
• [ ] Sessions
• [ ] In-App Messaging
• [ ] REST API requests
• [ ] Public API changes

Checklist

Overview

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

Testing

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

Final pass

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

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