Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

platform/android/thread_jandroid.h

@@ -35,7 +35,7 @@
 	@author Juan Linietsky <reduzio@gmail.com>
 */
 
-#include "os/thread.h"
+#include "core/os/thread.h"
 #include <jni.h>
 #include <pthread.h>
 #include <sys/types.h>