libbinder: support server-specific session

When multiple clients connect to a server, we need a way to tell these
clients apart. Having a per-client root object is the easiest way to do
this (the alternative, using getCalling* like is used in binder, isn't
so great because it requires global/thread local place, but given that
many RpcSession objects can be created, and these can also be used in
conjunction with kernel binder, it is complicated figuring out exactly
where to call getCalling*).

Bug: 199259751
Test: binderRpcTest
Change-Id: I5727db618b5ea138bfa19e75ed915f6a6991518e