Move BufferNode to libbufferhubservice

BufferNode is a server-side class and should be located in the service
folder. Also, making the old bufferhubd service depending on
libbufferhubservice could solve the dependency problem.

Test: "atest buffer_node-test", "atest buffer_hub-test" passed.
Bug: 118893702
Change-Id: I5fad37d3c0475d6cd4f4e0ed17f911b6777a6868