Update usb_connected() to support new gadget driver

// Return true if USB is connected.

static int usb_connected() {

    int fd = open("/sys/class/switch/usb_connected/state", O_RDONLY);

    int fd = open("/sys/class/android_usb/android0/state", O_RDONLY);

    if (fd < 0) {

        printf("failed to open /sys/class/switch/usb_connected/state: %s\n",

        printf("failed to open /sys/class/android_usb/android0/state: %s\n",

               strerror(errno));

        return 0;

    }

    char buf;

    int connected = (read(fd, &buf, 1) == 1) && (buf == '1');

    /* USB is connected if android_usb state is CONNECTED or CONFIGURED */

    int connected = (read(fd, &buf, 1) == 1) && (buf == 'C');

    if (close(fd) < 0) {

        printf("failed to close /sys/class/switch/usb_connected/state: %s\n",

        printf("failed to close /sys/class/android_usb/android0/state: %s\n",

               strerror(errno));

    }

    return connected;