Fix ClassDB API portability with some android and editor classes

- `EditorNavigationMeshGenerator` was being registered as part of the Core API,
even after d3f48f88bb. We must make sure to
set Editor as the current ClassDB API type before creating an instance.

- The `VisualScriptEngineSingleton.constant` property has a property hint string
that's different between tools and non-tools builds. This commit makes the
hint string to no longer be set in `_bind_methods`, and to instead set it in
`_validate_property`. This way it's ignored when calculating the API hash.

- `JavaClassWrapper` is now registered in ClassDB on all platforms,
using a dummy implementation on platforms other than Android.
This fixes API portability between Android and other platforms.

- Updated `--class-db-json` command to ignore non-virtual methods that start
with an underscore (see: 4be87c6016).

modules/mono/class_db_api_json.cpp

@@ -71,6 +71,13 @@ void class_db_api_to_json(const String &p_output_file, ClassDB::APIType p_api) {
 
 			while ((k = t->method_map.next(k))) {
 
+				String name = k->operator String();
+
+				ERR_CONTINUE(name.empty());
+
+				if (name[0] == '_')
+					continue; // Ignore non-virtual methods that start with an underscore
+
 				snames.push_back(*k);
 			}