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/jni_singleton.h

@@ -33,8 +33,9 @@
 
 #include "core/config/engine.h"
 #include "core/variant/variant.h"
+
 #ifdef ANDROID_ENABLED
-#include "platform/android/jni_utils.h"
+#include "jni_utils.h"
 #endif
 
 class JNISingleton : public Object {