feat(audio): coordinate Android audio focus and auto-yield on system interruptions

[yanwuu]

Summary

Resolves #36 by introducing Android system AudioFocus management and automatic ducking/muting for CarPlay audio playback on Android head units.

Problem

In vehicle environments, native infotainment components (such as Bluetooth telephone calls, native FM/AM radio, backup radar chimes, or native voice assistants) dynamically request Android audio focus.

Previously, xcertplay did not participate in Android's AudioFocus lifecycle. When a native audio event occurred:

• CarPlay media audio kept playing at full volume, colliding with phone calls or native alerts.
• On some automotive ROMs, unhandled focus loss resulted in unexpected audio subsystem resets or connection dropouts.

Solution

1. AudioFocusCoordinator:
   • Manages audio focus requests using AudioFocusRequest based on active stream priority:
     • MEDIA -> AUDIOFOCUS_GAIN
     • PHONE -> AUDIOFOCUS_GAIN_TRANSIENT
     • ASSISTANT -> AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK
     • NAVIGATION -> Coexists without claiming exclusive focus.
2. Audio Ducking and Auto-Yield (Muting):
   • Ducking: Automatically attenuates media volume to 20% (0.2f) upon receiving AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK.
   • Auto-Yield: Automatically mutes media volume to 0% (0.0f) upon AUDIOFOCUS_LOSS_TRANSIENT (e.g. during incoming/outgoing car Bluetooth phone calls or native car voice interruptions).
   • Restoration: Restores full volume (100%) upon AUDIOFOCUS_GAIN.
3. Settings & Persistence:
   • Added Audio focus and Mute media on temporary focus loss toggles in Settings -> Audio section (persisted in AirPlayPersistence).
   • Defaults to enabled for optimal automotive experience, while allowing users on non-compliant head units to disable it if needed.
4. Unit Tests:
   • Added AudioFocusCoordinatorTest covering channel focus priorities and volume constants.

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中文说明

问题背景：
在车机场景下，车载蓝牙电话、原车调频广播、倒车雷达提示音以及车载语音助手会随时请求 Android 系统音频焦点。此前 xcertplay 未接入 Android 音频焦点监听，导致在蓝牙来电或原车播报时，CarPlay 音乐依旧全音量播放，发生声音撞车或在部分车机上引发异常（对应 Issue #36）。

解决方案：

1. 音频焦点调度器 (AudioFocusCoordinator)：根据通道类型动态申请对应级别的音频焦点（媒体流申请长期焦点，通话/语音申请临时焦点，导航流不独占焦点）。
2. 自动压音与通话避让静音：
   • 收到短暂可降音信号（LOSS_TRANSIENT_CAN_DUCK）时，自动压低 CarPlay 音乐音量至 20%。
   • 收到临时失去焦点信号（LOSS_TRANSIENT，如原车蓝牙电话接通）时，自动将媒体流静音（0%），电话挂断并重获焦点（GAIN）后自动恢复 100% 音量。
3. 配置开关与持久化：在设置菜单音频板块新增对应开关与说明，并默认开启，保证即开即用与良好兼容性。