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/test/test_base.cpp

@@ -21,6 +21,7 @@
 #include "log.h"
 #include "oec_device_features.h"
 #include "oec_test_data.h"
+#include "platform.h"
 #include "properties.h"
 #include "test_printers.h"
 #include "url_request.h"