[JAVA] Reduce heap usage for variable width vectors

[asfimport]

This is a follow up to ARROW-1807. The same changes need to be done for variable len vectors too.

Also, the default value for initial allocations (4096) causes a lot of wastage, and needs to be changed (to 3970).

Reporter: Pindikura Ravindra / @pravindra
Assignee: Pindikura Ravindra / @pravindra

PRs and other links:

• GitHub Pull Request #3298

_{Note: This issue was originally created as ARROW-4147. Please see the migration documentation for further details.}