sqlite default filename is not Windows compatible

File `model_search/metadata/ml_metadata_db.py`, lines 102-103 are:
```
        self._connection_config.sqlite.filename_uri = (
            "/tmp/filedb-%d" % random.randint(0, 1000000))
```

`/tmp/...` is of course not a valid path for Windows. 
Can you update this path such that it chooses a valid path when running on Windows?