Fix Live Activities methods were not working due to bridge error

[nan-li]

Description

One Line Summary

Fix the Live Activities enter and exit methods were not working by registering the Live Activities class with Flutter.

Details

Motivation

Fix error

Unhandled Exception: MissingPluginException(No implementation found for method 
OneSignal#enterLiveActivity on channel OneSignal#liveactivities)

Scope

Bug fix that didn't register the OSFlutterLiveActivities class.

Testing

Unit testing

Manual testing

• Physical iphone 13 with ios 17.2
• Call OneSignal.LiveActivities.enterLiveActivity("activityId", "token") and OneSignal.LiveActivities.exitLiveActivity("activityId")
• Before PR: encounter that error
• After PR: See the native enter and exit requests get logged and receive successful network response

Affected code checklist

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

Checklist

Overview

• [x] I have filled out all REQUIRED sections above
• [x] PR does one thing
• [ ] 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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