Support IAccessor in libbinder for RPC services

This cl sets up preconnected RPC binder for services launched with
IAccessor as a proxy.

Bug: 338541373
Test: m
Test: atest vm_accessor_test
Change-Id: Ic54732980778bc9ba8fec3395a0e98d336fea440
diff --git a/libs/binder/RpcSession.cpp b/libs/binder/RpcSession.cpp
index 16a7f9f..21dc5c1 100644
--- a/libs/binder/RpcSession.cpp
+++ b/libs/binder/RpcSession.cpp
@@ -801,6 +801,10 @@
     return true;
 }
 
+void RpcSession::setSessionSpecificRoot(const sp<IBinder>& sessionSpecificRoot) {
+    mSessionSpecificRootObject = sessionSpecificRoot;
+}
+
 sp<RpcSession::RpcConnection> RpcSession::assignIncomingConnectionToThisThread(
         std::unique_ptr<RpcTransport> rpcTransport) {
     RpcMutexLockGuard _l(mMutex);