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
diff --git a/libs/binder/RpcSession.cpp b/libs/binder/RpcSession.cpp
index e8f984c..886a2e9 100644
--- a/libs/binder/RpcSession.cpp
+++ b/libs/binder/RpcSession.cpp
@@ -29,6 +29,7 @@
 #include <android-base/hex.h>
 #include <android-base/macros.h>
 #include <android_runtime/vm.h>
+#include <binder/BpBinder.h>
 #include <binder/Parcel.h>
 #include <binder/RpcServer.h>
 #include <binder/RpcTransportRaw.h>
@@ -226,6 +227,10 @@
                              sp<RpcSession>::fromExisting(this), reply, flags);
 }
 
+status_t RpcSession::sendDecStrong(const BpBinder* binder) {
+    return sendDecStrong(binder->getPrivateAccessor().rpcAddress());
+}
+
 status_t RpcSession::sendDecStrong(uint64_t address) {
     ExclusiveConnection connection;
     status_t status = ExclusiveConnection::find(sp<RpcSession>::fromExisting(this),