RpcServer: be thread safe

It's tempting to add an RpcServerConfig object, but we would also need
to allocate it on the heap. After some consideration, we don't really
care if a server changes dynamically. So, lifting all restrictions.

Bug: 185167543
Test: N/A
Change-Id: I134813203d0a2d8256ed88c6e59b4bb8d0bafc72