Fix permission not updating until start of session

[emawby]

Description

One Line Summary

OneSignal.Notification.permission value is now updated immediately when the permission value changes.

Details

Fixes #734

The permission field was not marked as async so we cannot use await _channel.invokeMethod("OneSignal#permission");
Instead I have added ourselves as a permission observer that will update the _permission boolean whenever the observer fires.

Motivation

bug fix

Scope

notification permissions

Testing

Manual testing

Tested on iOS device

Affected code checklist

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

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• 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.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

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