AvroUtils is converting incorrectly LogicalType Timestamps from long into Joda DateTimes

Copied from the mailing list report:

I think the method AvroUtils.toBeamSchema has a not expected side effect.
I found out that, if you invoke it and then you run a pipeline of GenericRecords containing a timestamp (l tried with logical-type timestamp-millis), Beam converts such timestamp from long to org.joda.time.DateTime. Even if you don't apply any transformation to the pipeline.
Do you think it's a bug?

More details on how to reproduce here:

https://lists.apache.org/thread.html/r43fb2896e496b7493a962207eb3b95360abc30b9d091b26f110264d0%40%3Cuser.beam.apache.org%3E

Imported from Jira [BEAM-9863](https://issues.apache.org/jira/browse/BEAM-9863). Original Jira may contain additional context.
Reported by: iemejia.

Start with AvroUtils.toBeamSchema and reproduce the linked mailing-list report using GenericRecords with a timestamp-millis logical type. Compare the record before and after invoking the method, and confirm whether the timestamp remains a long rather than becoming a Joda DateTime without a pipeline transformation.

Written by the indexing model from the issue text.