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/api/api.cpp

@@ -30,10 +30,11 @@
 
 #include "api.h"
 
-#include "core/config/engine.h"
 #include "java_class_wrapper.h"
 #include "jni_singleton.h"
 
+#include "core/config/engine.h"
+
 #if !defined(ANDROID_ENABLED)
 static JavaClassWrapper *java_class_wrapper = nullptr;
 #endif