feat(audio): AudioManager audio session management

[hiroshihorie]

What

First-class, process-wide audio session control on AudioManager — replacing session/routing behavior that was scattered across Hardware, track/audio_management.dart, and implicit native defaults.

API

Typed session configuration — AudioSessionOptions with communication() / media() presets and per-platform escape hatches:

await AudioManager.instance.setAudioSessionOptions(
  const AudioSessionOptions.communication(),
);

• AppleAudioSessionConfiguration: category / mode / category options
• AndroidAudioSessionConfiguration: audio mode, focus mode, stream type, audio attributes usage/content types

Automatic vs manual management — AudioSessionManagementMode. Default automatic preserves today's behavior (the SDK tracks local/remote audio presence and configures the session). manual lets apps that own their audio lifecycle drive it themselves.

Android session backend — a new AudioSwitch-based LKAudioSwitchManager in the native plugin handles device routing, audio focus acquisition/release, and communication-device management, driven by configureAndroidAudioSession / stopAndroidAudioSession channel methods.

Unified speakerphone/routing — setSpeakerphoneOn, preferSpeakerOutput, forceSpeakerOutput move to AudioManager with consistent Apple (overrideOutputAudioPort) and Android (AudioSwitch) implementations.

Compatibility

Hardware's audio members become deprecated forwarders to AudioManager — migration, not removal. Room uses AudioManager internally; no behavior change for apps that touch nothing.

Why AudioManager

Platform audio sessions are global to the app process — they cannot be per-Room or per-track. This is the second half of making AudioManager the home for process-wide audio: #1107 added the engine-wide processing state read-back, this adds session/routing control — mirroring the Swift SDK's AudioManager.

Notes

• Rebased on main post-feat(audio): runtime audio processing options and engine-wide state read-back #1107: the two AudioManager surfaces are unified in one class.
• Includes unit tests for session option serialization (8 passing).
• Changeset entry included (minor type="added").