Style: Harmonize header includes in platform ports

This applies our existing style guide, and adds a new rule to that style
guide for modular components such as platform ports and modules:

Includes from the platform port or module should be included with relative
paths (relative to the root folder of the modular component, e.g.
`platform/linuxbsd/`), in their own section before Godot's "core" includes.

The `api` and `export` subfolders also need to be handled as self-contained
(and thus use relative paths for their "local" includes) as they are all
compiled for each editor platform, without necessarily having the api/export
matching platform folder in the include path.
E.g. the Linux editor build will compile `platform/android/{api,export}/*.cpp`
and those need to use relative includes for it to work.

platform/android/string_android.h

@@ -31,8 +31,10 @@
 #ifndef STRING_ANDROID_H
 #define STRING_ANDROID_H
 
-#include "core/string/ustring.h"
 #include "thread_jandroid.h"
+
+#include "core/string/ustring.h"
+
 #include <jni.h>
 
 /**