Respect duration since last message trigger on app close and relaunch

[emawby]

Description

One Line Summary

This PR fixes a bug where we would not respect the duration since last message trigger if the app was restarted.

Details

When an IAM is dismissed we persist the time stamp as a string to OneSignal shared userdefaults. When the OSMessagingController is initialized we read from userDefaults and set the triggerController's timeSinceLastMessage property

Motivation

bug fix

Testing

Unit testing

We do not have the infrastructure to test this kind of IAM persisting in unit tests currently.

Manual testing

tested on iOS device with the proper IAM trigger.

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 

[jkasten2]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @nan-li)