Platform check the notification system to unblock Android

[HeyImChris]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

Android doesn't need this so ifdef around their platform. Apple's TargetConditionals.h defines TARGET_OS_MAC to be true for iOS and OSX.

Test Plan

If CI builds it should be all set.

[pull-bot]

	Warnings
⚠️	❔ Base Branch - The base branch for this PR is something other than master. Are you sure you want to merge these changes into a stable release? If you are interested in backporting updates to an older release, the suggested approach is to land those changes on master first and then cherry-pick the commits into the branch for that release. The Releases Guide has more information.

Messages

📖

📋 Missing Changelog - Can you add a Changelog? To do so, add a "## Changelog" section to your PR description. A changelog entry has the following format: [CATEGORY] [TYPE] - Message. Details CATEGORY may be: General macOS iOS Android JavaScript Internal (for changes that do not need to be called out in the release notes) TYPE may be: Added, for new features. Changed, for changes in existing functionality. Deprecated, for soon-to-be removed features. Removed, for now removed features. Fixed, for any bug fixes. Security, in case of vulnerabilities. MESSAGE may answer "what and why" on a feature level. Use this to briefly tell React Native users about notable changes.

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