AudioManger和AudioSystem的关系

    // This section re-defines new output device constants from AudioSystem, because the AudioSystem
    // class is not used by other parts of the framework, which instead use definitions and methods
    // from AudioManager. AudioSystem is an internal class used by AudioManager and AudioService.

    /** @hide
     * The audio device code for representing "no device." */
    public static final int DEVICE_NONE = AudioSystem.DEVICE_NONE;
    /** @hide
     *  The audio output device code for the small speaker at the front of the device used
     *  when placing calls.  Does not refer to an in-ear headphone without attached microphone,
     *  such as earbuds, earphones, or in-ear monitors (IEM). Those would be handled as a
     *  {@link #DEVICE_OUT_WIRED_HEADPHONE}.
     */
    public static final int DEVICE_OUT_EARPIECE = AudioSystem.DEVICE_OUT_EARPIECE;
    /** @hide
     *  The audio output device code for the built-in speaker */
    public static final int DEVICE_OUT_SPEAKER = AudioSystem.DEVICE_OUT_SPEAKER;
    /** @hide
     * The audio output device code for a wired headset with attached microphone */
    public static final int DEVICE_OUT_WIRED_HEADSET = AudioSystem.DEVICE_OUT_WIRED_HEADSET;
    /** @hide
     * The audio output device code for a wired headphone without attached microphone */
    public static final int DEVICE_OUT_WIRED_HEADPHONE = AudioSystem.DEVICE_OUT_WIRED_HEADPHONE;
    /** @hide
     * The audio output device code for generic Bluetooth SCO, for voice */
    public static final int DEVICE_OUT_BLUETOOTH_SCO = AudioSystem.DEVICE_OUT_BLUETOOTH_SCO;
    /** @hide
     * The audio output device code for Bluetooth SCO Headset Profile (HSP) and
     * Hands-Free Profile (HFP), for voice
     */
    public static final int DEVICE_OUT_BLUETOOTH_SCO_HEADSET =
            AudioSystem.DEVICE_OUT_BLUETOOTH_SCO_HEADSET;
    /** @hide
     * The audio output device code for Bluetooth SCO car audio, for voice */
    public static final int DEVICE_OUT_BLUETOOTH_SCO_CARKIT =
            AudioSystem.DEVICE_OUT_BLUETOOTH_SCO_CARKIT;

之前对二者的关系不是特别的清楚. 现在搞清楚.

通过这段注释 我们可以很清晰的知道

 // This section re-defines new output device constants from AudioSystem, because the AudioSystem
    // class is not used by other parts of the framework, which instead use definitions and methods
    // from AudioManager. AudioSystem is an internal class used by AudioManager and AudioService.

AudioSystem 是 AudioManager and AudioService. 的  internal class 内部类, 不对别的模块提供支持.