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ARROW-6145: [Java] UnionVector created by MinorType#getNewVector could not keep field type info properly

Related to [ARROW-6145](https://issues.apache.org/jira/browse/ARROW-6145).
When I worked for other items, I found UnionVector created by VectorSchemaRoot#create(Schema schema, BufferAllocator allocator) could not keep field type info properly. For example, if we set metadata in Field in schema, we could not get it back by UnionVector#getField.

This is mainly because MinorType.Union.getNewVector did not pass FieldType to vector and UnionVector#getField create a new Field which cause inconsistent.

Closes #5023 from tianchen92/ARROW-6145 and squashes the following commits:

a11c4e1b1 <tianchen>  UnionVector created by MinorType#getNewVector could not keep field type info properly

Authored-by: tianchen <niki.lj@alibaba-inc.com>
Signed-off-by: Praveen <praveen@dremio.com>