factory_device_data
===================
Reads some fields from the device data.

Function Arguments
------------------
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   :align: left

   * - Name
     - Type
     - Description

   * - device_data_keys
     - list
     - A list of the field names of the device data field probe.

   * - probed_result_keys
     - list, None
     - (optional; default: ``None``) A list of output field names corresponding to each probed device data field.  Each element can be either a string of the new field name or ``None`` to use the original field name.  Leaving this argument empty is a syntax-sugar for ``[None, None, ...]``.

Description
-----------
This probe function is essentially an adapter for the probe-tool user to
access the factory device data.  It loads the data via the
``cros.factory.test.device_data`` module and outputs the specified fields
as the probed results.

Examples
--------

The following probe statement probes the serial numbers from the device::

  {
    "serial_number": {
      "from_device_data": {
        "eval": {
          "factory_device_data": {
            "device_data_keys": [
              "serials.serial_number",
              "serials.mlb_serial_number"
            ],
            "probed_result_keys": [
              "device_sn",
              "mlb_sn"
            ]
          }
        }
      }
    }
  }

The statement instructs the probe framework to load ``serials.serial_number``
and ``serials.mlb_serial_number`` and output them as the value of the fields
``device_sn`` and ``mlb_sn`` correspondingly.  Therefore, assume the device
data contains the following fields::

  serials.serial_number: "aabbcc"
  serials.mlb_serial_number: "ddeeff"

Then the corresponding output will be::

  {
    "serial_number": [
      {
        "name": "from_device_data",
        "values": {
          "device_sn": "aabbcc",
          "mlb_sn": "ddeeff"
        }
      }
    ],
  }