state: Add Available state to Memory

- Map `Available` to Redfish `UnavailableOffline` for Status.State when
  resource is present but not available

Tested:
- Unit tests passed
- Redfish Service Validator passed
- Request
```
curl -k -v https://${bmc}/redfish/v1/Systems/system/Memory/dimm3
```
Results in
```
{
  "@odata.id": "/redfish/v1/Systems/system/Memory/dimm3",
  "@odata.type": "#Memory.v1_12_0.Memory",
  ...
  "Status": {
    "Health": "OK",
    "State": "Enabled"
  }
  ...
```
`UnavailableOffline` if not available but present
```
  "Status": {
    "Health": "OK",
    "State": "UnavailableOffline"
  }
```

Change-Id: I163095f013bc233a8eb7de3894ef950a75d93379
Signed-off-by: Justin Nguyen <justinnanguyen@gmail.com>