[Java] Move ArrowBuf into the Arrow package

[asfimport]

After ARROW-7505 and ARROW-7935 are done, we are ready to move ArrowBuf into Arrow's package, and make it independent of Netty library.

Reporter: Liya Fan / @liyafan82
Assignee: Liya Fan / @liyafan82

Related issues:

• [Java] Remove Netty references from ArrowBuf and move Allocator out of vector package (is a child of)

PRs and other links:

• GitHub Pull Request #6729

_{Note: This issue was originally created as ARROW-8229. Please see the migration documentation for further details.}