libbinder: add makeDefaultRpcTransportCtxFactory

Add a new OS-specific function that creates a new
instance of the default RpcTransportCtxFactory.
This is needed because Android and Trusty have
different default transports: RpcTransportRaw and
RpcTransportTipcTrusty, respectively.

Bug: 230135749
Test: presubmit
Change-Id: I4abd443fe9a08c1fa0cc41dfca7ef1cdb69fe0fb
diff --git a/libs/binder/RpcSession.cpp b/libs/binder/RpcSession.cpp
index 8ddfa93..eee28d6 100644
--- a/libs/binder/RpcSession.cpp
+++ b/libs/binder/RpcSession.cpp
@@ -68,7 +68,7 @@
 
 sp<RpcSession> RpcSession::make() {
     // Default is without TLS.
-    return make(RpcTransportCtxFactoryRaw::make());
+    return make(makeDefaultRpcTransportCtxFactory());
 }
 
 sp<RpcSession> RpcSession::make(std::unique_ptr<RpcTransportCtxFactory> rpcTransportCtxFactory) {