[Java][FlightRPC] FlightServer can leak during cleanup

[jduo]

Describe the bug, including details regarding any error messages, version, and platform.

In #38318 , a few Flight tests were flakey due to occasionally leaking during cleanup. The leaks were coming from the server's allocator specifically. A workaround is to cleanup child allocators, however it would be best to determine the root cause of the leak and not have to manually clean-up child allocators.

Component(s)

FlightRPC, Java