[R0]默认MTP模式

\frameworks\base\services\usb\java\com\android\server\usb\UsbDeviceManager.java
1、

@Override
public void handleMessage(Message msg) {
    switch (msg.what) {
        case MSG_UPDATE_STATE:
            ……
                // restore defaults when USB is disconnected
                if (!mScreenLocked
                    && mScreenUnlockedFunctions != UsbManager.FUNCTION_NONE) {
                    setScreenUnlockedFunctions();
                } else {
                    // 这里改MTP
                    //setEnabledFunctions(UsbManager.FUNCTION_NONE, false);
                    setEnabledFunctions(UsbManager.FUNCTION_MTP, false);
                }
            ……   
        break;

2、

protected void finishBoot() {
    ……
        if (!mScreenLocked
            && mScreenUnlockedFunctions != UsbManager.FUNCTION_NONE) {
            setScreenUnlockedFunctions();
        } else {
            // 这里改MTP
            //setEnabledFunctions(UsbManager.FUNCTION_NONE, false);
            setEnabledFunctions(UsbManager.FUNCTION_MTP, false);
        }
    ……
}