[User Model] Add plist option to override gray overlay to In App Messages

[jennantilla]

Description

One Line Summary

Add plist option to override gray overlay to In App Messages

Details

In center modal and full screen (both centered) IAMs, a dark gray background is applied. Customers would like the option to disable this overlay.

With this PR developers can now use boolean plist setting OneSignal_in_app_message_hide_gray_overlay to toggle if they wish to remove the overlay from center modal and full screen IAMs.

Examples with the overlay

Examples without the overlay

Scope

New and existing In App Message visual behavior

Testing

Manual testing

Tested on center and full IAM types with a iPhone 15 simulator running iOS 17.2 without plist, with plist option to YES, and with plist option to NO.

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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This change is 

[nan-li]

Tested on my end