[Integration][Java] Fix map type to allow non-standard field names

Java should support the integration test added in ARROW-7173.

**Reporter**: [Antoine Pitrou](https://issues.apache.org/jira/browse/ARROW-8715) / @pitrou

_{**Note**: *This issue was originally created as [ARROW-8715](https://issues.apache.org/jira/browse/ARROW-8715). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the Java integration test added in ARROW-7173 and trace the map type handling it exercises. Confirm the implementation supports non-standard field names, then run that integration test; done means the Java integration passes for the ARROW-7173 case.

Written by the indexing model from the issue text.