The flutter/lifecycle channel is available in Android, iOS, web, but isn't implemented in desktop platforms. This issue tracks PR for each desktop platforms.
With this channel, framework can know whether an app window is visibled or focused.

This feature will unblock #87061.
Related: #30735

SystemChannels.lifecycle docs: https://api.flutter.dev/flutter/services/SystemChannels/lifecycle-constant.html
WidgetsBindingObserver.didChangeAppLifecycleState docs: https://api.flutter.dev/flutter/widgets/WidgetsBindingObserver/didChangeAppLifecycleState.html

• Add common StringMessage and StringMessageCodec: Win32: Add lifecycle channel support engine#29288
• Implement for Windows: Win32: Add lifecycle channel support engine#29288
• Implement for macOS:
• Implement for Linux: