feat: use last predicted stop as added trip headsign

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Summary of changes

Asana Ticket: 🍎 Adjust API headsign logic for ADDED trips

[Please include a brief description of what was changed]

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Coverage of commit 3e27228

Summary coverage rate:
  lines......: 89.0% (4216 of 4736 lines)
  functions..: 70.2% (2285 of 3257 functions)
  branches...: no data found

Files changed coverage rate:
                                                                        |Lines       |Functions  |Branches    
  Filename                                                              |Rate     Num|Rate    Num|Rate     Num
  ============================================================================================================
  apps/state/lib/state/trip/added.ex                                    | 100%     32|72.7%    44|    -      0

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Comparison from today's RL disruption with service suspended between Harvard and Broadway:
prod:

{
  "data": {
    "attributes": {
      "bikes_allowed": 0,
      "block_id": null,
      "direction_id": 0,
      "headsign": "Braintree",
      "name": "",
      "revenue": "REVENUE",
      "wheelchair_accessible": 1
    },
    "id": "ADDED-1582142747",
    "links": {
      "self": "/trips/ADDED-1582142747"
    },
    "relationships": {
      "route": {
        "data": {
          "id": "Red",
          "type": "route"
        }
      },
      "route_pattern": {
        "data": {
          "id": "Red-3-0",
          "type": "route_pattern"
        }
      },
      "service": {
        "data": null
      },
      "shape": {
        "data": {
          "id": "canonical-933_0009",
          "type": "shape"
        }
      }
    },
    "type": "trip"
  },
  "jsonapi": {
    "version": "1.0"
  }
}

dev-green:

{
  "data": {
    "attributes": {
      "bikes_allowed": 0,
      "block_id": null,
      "direction_id": 0,
      "headsign": "Harvard",
      "name": "",
      "revenue": "REVENUE",
      "wheelchair_accessible": 1
    },
    "id": "ADDED-1590682122",
    "links": {
      "self": "/trips/ADDED-1590682122"
    },
    "relationships": {
      "route": {
        "data": {
          "id": "Red",
          "type": "route"
        }
      },
      "route_pattern": {
        "data": {
          "id": "Red-3-0",
          "type": "route_pattern"
        }
      },
      "service": {
        "data": null
      },
      "shape": {
        "data": {
          "id": "canonical-933_0009",
          "type": "shape"
        }
      }
    },
    "type": "trip"
  },
  "jsonapi": {
    "version": "1.0"
  }
}

(Added trip IDs are not consistent between environments. I found these to compare by downloading the Prod/Green RT feeds and finding the predictions for vehicle R-547FBF60)