libbinder: RpcSession - hide RPC address format

FAQ for this change.

- Why does RpcSession still have sendDecStrong(uint64_t)?

    This is used by RpcState to send dec strongs on local (non-BpBinder)
    binders. This is needed because if a client has the last strong ref
    of a binder which is local to the corresponding server, the client
    needs to transfer ownership of this binder to the server using the
    call.

- Why is RpcSession::sendDecStrong(BpBinder) public?

    It may be useful for testing, especially in situations where
    ~BpBinder would be hard to call.

- Why does BpBinder have a private accessor, but RpcState is friends
  with some other Rpc* classes?

    I would like to have as few friends as possible (in regards to C++
    classes!), but it takes quite a bit of boilerplate to avoid this. I
    want to avoid the Rpc stuff introspecting into the standard binder
    stuff, but I'm not currently concerned about their interactions
    between each other.

Bug: 167966510
Test: binderRpcTest
Change-Id: I89e4de2e3a042a4531e2fd41deeb9bab1c11ff94