libbinder: RpcSession exposes sp<RpcServer>

This object guarantees a strong pointer IFF it is associated with a
server. The wp<> return type here previously was for convenience (idk?)
but users of it shouldn't be concerned with the underlying memory
situation.

Bug: 167966510
Test: binderRpcTest
Change-Id: I6578c3a4246e1bd07f7697c11d4b56899b50245b