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/wvcrc.cpp

@@ -4,7 +4,7 @@
 //
 // Compute CRC32 Checksum. Needed for verification of WV Keybox.
 //
-#include <arpa/inet.h>
+#include "platform.h"
 #include "wvcrc32.h"
 
 namespace wvoec {