A common platform interface for the audio_service plugin.

This interface allows platform-specific implementations of the audio_service plugin, as well as the plugin itself, to ensure they are supporting the same interface.

To implement a new platform-specific implementation of audio_service, extend AudioServicePlatform with an implementation that performs the platform-specific behavior, and when you register your plugin, set the default AudioServicePlatform by calling AudioServicePlatform.instance = MyPlatformAudioService().

Strongly prefer non-breaking changes (such as adding a method to the interface) over breaking changes for this package.

See https://flutter.dev/go/platform-interface-breaking-changes for a discussion on why a less-clean interface is preferable to a breaking change.