[5.0.0] Make subscribers in EventProducer synchronized

[jennantilla]

Description

One Line Summary

Address ConcurrentModificationException crash by making subscribers in EventProducer synchronized

Details

Motivation

Customer issue of ConcurrentModificationException reported by Firebase Crashlytics while registering for push notifications.

Scope

Crash occurs while iterating over subscribers list in EventProducer.

Testing

Unit testing

No unit testing used for this fix. Future unit test could be written to create a multithreading environment.

Manual testing

Unable to reproduce original crash. Tested branch builds and push notifications registration is successful.

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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