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/util/include/file_store.h

@@ -7,12 +7,13 @@
 #ifndef WVCDM_UTIL_FILE_STORE_H_
 #define WVCDM_UTIL_FILE_STORE_H_
 
-#include <unistd.h>
+#include <cstdint>
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "disallow_copy_and_assign.h"
+#include "platform.h"
 #include "util_common.h"
 
 namespace wvcdm {