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).

platform/android/api/api.cpp

@@ -0,0 +1,56 @@
+#include "api.h"
+
+#include "core/engine.h"
+#include "java_class_wrapper.h"
+
+#if !defined(ANDROID_ENABLED)
+static JavaClassWrapper *java_class_wrapper = NULL;
+#endif
+
+void register_android_api() {
+
+#if !defined(ANDROID_ENABLED)
+	java_class_wrapper = memnew(JavaClassWrapper); // Dummy
+#endif
+
+	ClassDB::register_class<JavaClass>();
+	ClassDB::register_class<JavaClassWrapper>();
+	Engine::get_singleton()->add_singleton(Engine::Singleton("JavaClassWrapper", JavaClassWrapper::get_singleton()));
+}
+
+void unregister_android_api() {
+
+#if !defined(ANDROID_ENABLED)
+	memdelete(java_class_wrapper);
+#endif
+}
+
+void JavaClassWrapper::_bind_methods() {
+
+	ClassDB::bind_method(D_METHOD("wrap", "name"), &JavaClassWrapper::wrap);
+}
+
+#if !defined(ANDROID_ENABLED)
+
+Variant JavaClass::call(const StringName &, const Variant **, int, Variant::CallError &) {
+	return Variant();
+}
+
+JavaClass::JavaClass() {
+}
+
+Variant JavaObject::call(const StringName &, const Variant **, int, Variant::CallError &) {
+	return Variant();
+}
+
+JavaClassWrapper *JavaClassWrapper::singleton = NULL;
+
+Ref<JavaClass> JavaClassWrapper::wrap(const String &) {
+	return Ref<JavaClass>();
+}
+
+JavaClassWrapper::JavaClassWrapper() {
+	singleton = this;
+}
+
+#endif