[JAVA] Vector should avoid reallocating validity buffer when necessary

[asfimport]

Vector always reallocates validity buffer in the case of all-null or non-null

https://github.com/apache/arrow/blob/master/java/vector/src/main/java/org/apache/arrow/vector/BaseFixedWidthVector.java#L511

https://github.com/apache/arrow/blob/master/java/vector/src/main/java/org/apache/arrow/vector/BitVectorHelper.java#L180

Ideally we should only allocate if the source was not provided

Reporter: Zhenyuan Zhao / @zzhao0

_{Note: This issue was originally created as ARROW-3222. Please see the migration documentation for further details.}