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/ref/src/oemcrypto_session.cpp

@@ -6,7 +6,6 @@
 //
 #include "oemcrypto_session.h"
 
-#include <arpa/inet.h>
 #include <assert.h>
 #include <string.h>
 #include <algorithm>
@@ -30,6 +29,7 @@
 #include "oemcrypto_key_ref.h"
 #include "oemcrypto_rsa_key_shared.h"
 #include "oemcrypto_types.h"
+#include "platform.h"
 #include "disallow_copy_and_assign.h"
 #include "string_conversions.h"
 #include "wvcrc32.h"