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/dir_access_jandroid.h

@@ -31,9 +31,11 @@
 #ifndef DIR_ACCESS_JANDROID_H
 #define DIR_ACCESS_JANDROID_H
 
+#include "java_godot_lib_jni.h"
+
 #include "core/io/dir_access.h"
 #include "drivers/unix/dir_access_unix.h"
-#include "java_godot_lib_jni.h"
+
 #include <stdio.h>
 
 /// Android implementation of the DirAccess interface used to provide access to