[core] Decouple Scheduler interface from actor model

So that it is possible to schedule normal `std::function` and use `mapbox::base::WeakPtr`.

platform/android/src/map_renderer.cpp

@@ -43,7 +43,7 @@ ActorRef<Renderer> MapRenderer::actor() const {
     return *rendererRef;
 }
 
-void MapRenderer::schedule(std::weak_ptr<Mailbox> scheduled) {
+void MapRenderer::schedule(std::function<void()> scheduled) {
     // Create a runnable
     android::UniqueEnv _env = android::AttachEnv();
     auto runnable = std::make_unique<MapRendererRunnable>(*_env, std::move(scheduled));