libbinder: Parcel::appendFrom better format check

(thanks jooyung@ for the catch here)

Even if we have two RPC parcels, they need to both be for the RPC
session. Otherwise, they might be at different versions (and we don't
really have a way to reconcile this).

Bug: 182939048
Test: binderRpcTest
Change-Id: I5180ebc681ccc79bf05a3fc0900a9b240e793a66