Add some platform abstractions.

(This is a merge of http://go/wvgerrit/70303)

This adds a platform.h file to abstract some of the differences
between Windows and POSIX platforms.  This includes ntohl, setenv,
and ssize_t.

Bug: 122953649
Test: Android Unit Tests
Change-Id: I3235f3f284b53d24d7365ff3f4a06dcd9b403697

libwvdrmengine/oemcrypto/test/oec_session_util.cpp

@@ -7,7 +7,6 @@
 
 #include "oec_session_util.h"
 
-#include <arpa/inet.h>  // needed for ntoh()
 #include <openssl/aes.h>
 #include <openssl/bio.h>
 #include <openssl/cmac.h>
@@ -32,6 +31,7 @@
 #include "oec_device_features.h"
 #include "oec_test_data.h"
 #include "oemcrypto_types.h"
+#include "platform.h"
 #include "string_conversions.h"
 
 using namespace std;