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ARROW-6343: [Java][Vector] Fix allocation helper.

- Allocation helper not passing value count when
  creating varchar vectors.

Closes #5186 from praveenbingo/ARROW-6343 and squashes the following commits:

95706c690 <Praveen> ARROW-6343:  Fix allocation helper.

Authored-by: Praveen <praveen@dremio.com>
Signed-off-by: Praveen <praveen@dremio.com>