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ARROW-7534: [Java] Create a new java/contrib module

To better clarify the status of java sub-modules, create a contrib module and move the following modules underneath it.

- algorithm
- adapter
- plasma

Closes #6812 from liyafan82/fly_0402_mv

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>