Add opt-in accessibility artifact record and verify support

[colinmarsch]

This change adds opt-in functionality (behind a gradle configuration property) to handle recording and verifying an accessibility hierarchy json string artifact for the #2239 feature.

Example output as follows:

[
  {
    "id": "Number 1",
    "beforeElementId": null,
    "afterElementId": "Number 2",
    "bounds": {
      "left": 57,
      "top": 57,
      "right": 215,
      "bottom": 616
    },
    "legendText": "Number 1"
  },
  {
    "id": "Number 2",
    "beforeElementId": "Number 1",
    "afterElementId": "Number 3",
    "bounds": {
      "left": 57,
      "top": 680,
      "right": 215,
      "bottom": 1239
    },
    "legendText": "Number 2"
  },
  {
    "id": "Number 3",
    "beforeElementId": "Number 2",
    "afterElementId": null,
    "bounds": {
      "left": 57,
      "top": 1303,
      "right": 215,
      "bottom": 1863
    },
    "legendText": "Number 3"
  }
]