Google RoT: remove global constant strings

These variables are replaced by local string literals, which has
better readability.

Tested: tested on real hardware.

GET: /google/v1/RootOfTrustCollection
{
  "@odata.id": "/google/v1/RootOfTrustCollection",
  "@odata.type": "#RootOfTrustCollection.RootOfTrustCollection",
  "Members": [
    {
      "@odata.id": "/google/v1/RootOfTrustCollection/Hoth"
    }
  ],
  "Members@odata.count": 1
}

GET /google/v1/RootOfTrustCollection/Hoth
{
  "@odata.id": "/google/v1/RootOfTrustCollection/Hoth",
  "@odata.type": "#RootOfTrust.v1_0_0.RootOfTrust",
  "Actions": {
    "#RootOfTrust.SendCommand": {
      "target": "/google/v1/RootOfTrustCollection/Hoth/Actions/RootOfTrust.SendCommand"
    }
  },
  "Description": "Google Root Of Trust",
  "Id": "Hoth",
  "Location": {
    "PartLocation": {
      "LocationType": "Embedded",
      "ServiceLabel": "Hoth"
    }
  },
  "Name": "Hoth",
  "Status": {
    "State": "Enabled"
  }
}

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I4c21eeb6a521b657bd9a8eb7394e7748d000ad52