AvroCoder is quite overconfident: it claims to be able to create a schema for things it certainly cannot

AvoCoder claims it can create a schema for:

```

public static class X {
public Thread t = Thread.currentThread();
}

```

The issue goes all the way to Avro, we think. To be investigated. Meanwhile, we need AvroCoder to say no to such things.

Imported from Jira [BEAM-2182](https://issues.apache.org/jira/browse/BEAM-2182). Original Jira may contain additional context.
Reported by: kenn.

Start by reproducing the Java example with AvroCoder and trace how schema generation handles the Thread field, including the path into Avro mentioned in the issue. Done means AvroCoder rejects this unsupported type instead of claiming it can create a schema.

Written by the indexing model from the issue text.