libbinder: separate OS-specific code from Utils.cpp

Separate the OS-specific functions setNonBlocking and
getRandomBytes from Utils.cpp into a separate OS-specific
OS.cpp file that other operating systems can override at
build time.

Bug: 224644083
Test: build Trusty
Change-Id: I2703fb81d4370b5f6264556ae57fdddb0573971a
diff --git a/libs/binder/RpcSession.cpp b/libs/binder/RpcSession.cpp
index 2d9c933..80f6a37 100644
--- a/libs/binder/RpcSession.cpp
+++ b/libs/binder/RpcSession.cpp
@@ -37,6 +37,7 @@
 #include <utils/String8.h>
 
 #include "FdTrigger.h"
+#include "OS.h"
 #include "RpcSocketAddress.h"
 #include "RpcState.h"
 #include "RpcWireFormat.h"