[core] Make the BackgroundScheduler a singleton

- Do not carry it over everywhere as parameter, it is a shared
  instance anyway and the lifecycle is pretty much the app lifecycle
  from the moment we instantiate a map.
- Rename to BackgroundScheduler because it is a Scheduler that will
  do tasks in the background, we don't make assumptions if it is a
  thread pool or a single thread.
- Most importantly, remove the dependency from `core` on `platform`.

platform/android/src/map_renderer.cpp

@@ -2,7 +2,6 @@
 
 #include <mbgl/renderer/renderer.hpp>
 #include <mbgl/gfx/backend_scope.hpp>
-#include <mbgl/util/shared_thread_pool.hpp>
 #include <mbgl/util/run_loop.hpp>
 
 #include <string>
@@ -23,7 +22,6 @@ MapRenderer::MapRenderer(jni::JNIEnv& _env,
         , pixelRatio(pixelRatio_)
         , programCacheDir(jni::Make<std::string>(_env, programCacheDir_))
         , localIdeographFontFamily(localIdeographFontFamily_ ? jni::Make<std::string>(_env, localIdeographFontFamily_) : optional<std::string>{})
-        , threadPool(sharedThreadPool())
         , mailbox(std::make_shared<Mailbox>(*this)) {
 }
 
@@ -175,7 +173,7 @@ void MapRenderer::onSurfaceCreated(JNIEnv&) {
 
     // Create the new backend and renderer
     backend = std::make_unique<AndroidRendererBackend>();
-    renderer = std::make_unique<Renderer>(*backend, pixelRatio, *threadPool, programCacheDir, localIdeographFontFamily);
+    renderer = std::make_unique<Renderer>(*backend, pixelRatio, programCacheDir, localIdeographFontFamily);
     rendererRef = std::make_unique<ActorRef<Renderer>>(*renderer, mailbox);
 
     // Set the observer on the new Renderer implementation