Add duplicate to BufferClient

Calling BufferClient::duplicate() will now return you an uint64_t token
stands for a specific BufferNode. In later CLs users could pass this
IPC-friendly token everywhere and use it to create a IBufferClient
linked to that BufferNode.

The token is derived from the mt19936_64 random algorithm. No same
tokens will be generated at the same time by checking the token_map_.
The map holds a weak_ptr so that if the BufferNode goes away by some
reason the map will not keep it, preventing from memory leak.

Add test case to test on the function.

Test: "atest buffer_hub_binder_service-test" passed.
Bug: 116681016
Change-Id: I54c34d9fabe882326faa141a6f0691419b740694