Android O 版本（Android 8.0） 修改默认音量

1，修改默认音量的位置为android/frameworks/base/media/java/android/media/AudioSystem.java

  public static int[] DEFAULT_STREAM_VOLUME = new int[] {
        4,  // STREAM_VOICE_CALL  普通通话音量
        7,  // STREAM_SYSTEM
        5,  // STREAM_RING      铃声
        11, // STREAM_MUSIC     音乐
        6,  // STREAM_ALARM     闹钟
        5,  // STREAM_NOTIFICATION 通知
        7,  // STREAM_BLUETOOTH_SCO 蓝牙耳机通话音量
        7,  // STREAM_SYSTEM_ENFORCED 
        11, // STREAM_DTMF
        11  // STREAM_TTS
    };

2，修改默认音量的最大值/最小值：android/frameworks/base/services/core/java/com/android/server/audio/AudioService.java

/** Maximum volume index values for audio streams */
    private static int[] MAX_STREAM_VOLUME = new int[] {
        5,  // STREAM_VOICE_CALL
        7,  // STREAM_SYSTEM
        7,  // STREAM_RING
        15, // STREAM_MUSIC
        7,  // STREAM_ALARM
        7,  // STREAM_NOTIFICATION
        15, // STREAM_BLUETOOTH_SCO
        7,  // STREAM_SYSTEM_ENFORCED
        15, // STREAM_DTMF
        15  // STREAM_TTS
    };

    /** Minimum volume index values for audio streams */
    private static int[] MIN_STREAM_VOLUME = new int[] {
        1,  // STREAM_VOICE_CALL
        0,  // STREAM_SYSTEM
        0,  // STREAM_RING
        0,  // STREAM_MUSIC
        0,  // STREAM_ALARM
        0,  // STREAM_NOTIFICATION
        1,  // STREAM_BLUETOOTH_SCO
        0,  // STREAM_SYSTEM_ENFORCED
        0,  // STREAM_DTMF
        0   // STREAM_TTS
    };