[Java] Make ComplexCopier agnostic of specific implementation of MapWriter, i.e UnionMapWriter

Making ComplexCopier independent of UnionMapWriter lets us use different implementations, like PromotableWriter instead. This helps us to copy map vector with a map value. Otherwise we get the following error:

ClassCast class org.apache.arrow.vector.complex.impl.PromotableWriter cannot be cast to class org.apache.arrow.vector.complex.impl.UnionMapWriter