Fail to connect to Azure Kinect with json configuration file

**Describe the bug**
Fail to connect to Azure Kinect with json configuration file.

**To Reproduce**
Steps to reproduce the behavior:
```python
config = o3d.io.AzureKinectSensorConfig()
o3d.io.write_azure_kinect_sensor_config("config.json", config)
config = o3d.io.read_azure_kinect_sensor_config('config.json')
sensor = o3d.io.AzureKinectSensor(config)
sensor.connect(0)
```

The Akinect.json is below:
```json
{
"camera_fps" : "K4A_FRAMES_PER_SECOND_30",
"color_format" : "K4A_IMAGE_FORMAT_COLOR_MJPG",
"color_resolution" : "K4A_COLOR_RESOLUTION_720P",
"depth_delay_off_color_usec" : "0",
"depth_mode" : "K4A_DEPTH_MODE_WFOV_2X2BINNED",
"disable_streaming_indicator" : "false",
"subordinate_delay_off_master_usec" : "0",
"synchronized_images_only" : "false",
"wired_sync_mode" : "K4A_WIRED_SYNC_MODE_STANDALONE"
}
```

See error:
```
[Open3D INFO] AzureKinectSensor::Connect
[Open3D INFO] sensor_index 0
[Open3D INFO] Serial number: 000647492212
[Open3D INFO] Firmware build: Rel
[Open3D INFO] > Color: 1.6.102
[Open3D INFO] > Depth: 1.6.75[6109.7]
[2020-05-15 09:38:27.929] [error] [t=5125] ../../../src/usbcommand/usbcommand.c (838): usb_cmd_write(). Write command(80000001) ended in failure, Command status 0x00000001
[2020-05-15 09:38:27.929] [error] [t=5125] ../../../src/color_mcu/color_mcu.c (284): usb_cmd_write( colormcu->usb_cmd, DEV_CMD_SET_SYS_CFG, (uint8_t *)&sync_config, sizeof(sync_config), NULL, 0) returned failure in colormcu_set_multi_device_mode()
[2020-05-15 09:38:27.929] [error] [t=5125] ../../../src/sdk/k4a.c (879): colormcu_set_multi_device_mode(device->colormcu, config) returned failure in k4a_device_start_cameras()
[Open3D WARNING] Runtime error: k4a_plugin::k4a_device_set_color_control() failed
False
```
**Expected behavior**
When I use the default configuration `config = o3d.io.AzureKinectSensorConfig()` directly without saving it into json file and read in again I'm able to connect to the camera and get frames.

**Screenshots**

**Environment (please complete the following information):**
- OS: Ubuntu 16.04
- Python version: 3.7
- Open3D version: 0.9
- Is this remote workstation?:no
- How did you install Open3D?: pip