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/cdm/core/src/initialization_data.cpp

@@ -4,7 +4,6 @@
 
 #include "initialization_data.h"
 
-#include <arpa/inet.h>
 #include <string.h>
 #include <sstream>
 
@@ -12,6 +11,7 @@
 #include "cdm_engine.h"
 #include "jsmn.h"
 #include "log.h"
+#include "platform.h"
 #include "properties.h"
 #include "string_conversions.h"
 #include "wv_cdm_constants.h"