libbinder: clear RpcTransportCtx for new sessions in RpcServer

RpcServer previously created new RpcSession instances
using the default transport. There is currently no code
in RpcSession that uses that transport for server sessions,
so this patch initializes the transport to nullptr instead.

Bug: 224644083
Test: atest binderRpcTest
Change-Id: Ie9fa8c9c758821614fb161b360a9935955e7e62b
diff --git a/libs/binder/RpcServer.cpp b/libs/binder/RpcServer.cpp
index 05e0f37..528341e 100644
--- a/libs/binder/RpcServer.cpp
+++ b/libs/binder/RpcServer.cpp
@@ -392,7 +392,7 @@
                 }
             } while (server->mSessions.end() != server->mSessions.find(sessionId));
 
-            session = RpcSession::make();
+            session = sp<RpcSession>::make(nullptr);
             session->setMaxIncomingThreads(server->mMaxThreads);
             if (!session->setProtocolVersion(protocolVersion)) return;