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/file_access_android.cpp

@@ -29,7 +29,7 @@
 /*************************************************************************/
 
 #include "file_access_android.h"
-#include "print_string.h"
+#include "core/print_string.h"
 
 AAssetManager *FileAccessAndroid::asset_manager = NULL;