Remove OneSignal gradle plugin from build.gradle files

[brismithers]

Description

One Line Summary

Remove the inclusion of the OneSignal gradle plugin from the example app.

Details

Motivation

The OneSignal gradle plugin for Android is designed to (1) ensure the compileSdkVersion is 31 or higher and (2) ensure (mostly) firebase dependent libraries in use by the app & onesignal are compatible. The dependent library compatibility problem has been less of an issue, making the plugin less useful. However inclusion of the plugin itself can sometimes cause side-effect issues during build.

As a result, including the OneSignal Gradle plugin is no longer considered standard when configuring an app to use the OneSignal SDK.

Scope

The OneSignal gradle plugin is no longer added into the build scripts, and is therefore no longer part of the build process.

Testing

Manual testing

As this only affects the build scripts, testing focused on the building of the example app and general functionality was tested within the Android emulator.

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 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @emawby and @nan-li)