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/OS.cpp b/libs/binder/OS.cpp
index cc4a03b..24ce2bb 100644
--- a/libs/binder/OS.cpp
+++ b/libs/binder/OS.cpp
@@ -17,6 +17,7 @@
 #include "OS.h"
 
 #include <android-base/file.h>
+#include <binder/RpcTransportRaw.h>
 #include <string.h>
 
 using android::base::ErrnoError;
@@ -58,4 +59,8 @@
     return OK;
 }
 
+std::unique_ptr<RpcTransportCtxFactory> makeDefaultRpcTransportCtxFactory() {
+    return RpcTransportCtxFactoryRaw::make();
+}
+
 } // namespace android