[Java] Fix splitAndTransfer for variable width vector

[asfimport]

When we splitAndTransfer a vector, the value count to set for the target vector should be equal to split length and not the value count of source vector. 

We have seen cases in operator slike FLATTEN and under low memory conditions, we end up allocating a lot more memory for the target vector because of using a large value in setValueCount after split and transfer is done.

Reporter: Siddharth Teotia / @siddharthteotia

_{Note: This issue was originally created as ARROW-2294. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.