How to describe this POST JSON request body in OpenAPI (Swagger)?

The most readable way to include a multi line scalar into YAML is by using the block literal style. This requires you to change your JSON example only by using indentation (which will be removed if you retrieve the value for the key):

.
.
produces:
  - application/json
example: |
  {
      "testapi": {
          "testapiContext": {
              "messageId": "kkkk8",
              "messageDateTime": "2014-08-17T14:07:30+0530"
     },
          "testapiBody": {
              "cameraServiceRq": {
                  "osType": "android",
                  "deviceType": "samsung555"
              }
          }
      }
  }
paths:
  /getCameraParameters:
.
.

(for clarity you can put an extra newline or two before the paths scalar key, they get clipped by default on the literal block style scalars.

I made it work with:

    post:
      consumes:
        - application/json
      produces:
        - application/json
        - text/xml
        - text/html
      parameters:
        - name: body
          in: body
          required: true
          schema:
            # Body schema with atomic property examples
            type: object
            properties:
              testapi:
                type: object
                properties:
                  messageId:
                    type: string
                    example: kkkk8
                  messageDateTime:
                    type: string
                    example: '2014-08-17T14:07:30+0530'
              testapiBody:
                type: object
                properties:
                  cameraServiceRq:
                    type: object
                    properties:
                      osType:
                        type: string
                        example: android
                      deviceType:
                        type: string
                        example: samsung555
            # Alternatively, we can use a schema-level example
            example:
              testapi:
                testapiContext:
                  messageId: kkkk8
                  messageDateTime: '2014-08-17T14:07:30+0530'
                testapiBody:
                  cameraServiceRq:
                    osType: android
                    deviceType: samsung555