[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`.

test/api/recycle_map.cpp

@@ -9,7 +9,6 @@
 #include <mbgl/style/sources/geojson_source.hpp>
 #include <mbgl/style/image.hpp>
 #include <mbgl/style/style.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/exception.hpp>
 #include <mbgl/util/geometry.hpp>
 #include <mbgl/util/geojson.hpp>
@@ -23,11 +22,9 @@ using namespace mbgl::style;
 TEST(API, RecycleMapUpdateImages) {
     util::RunLoop loop;
 
-    ThreadPool threadPool(4);
-
-    HeadlessFrontend frontend { 1, threadPool };
+    HeadlessFrontend frontend { 1 };
     auto map = std::make_unique<MapAdapter>(frontend, MapObserver::nullObserver(),
-                                         std::make_shared<StubFileSource>(), threadPool,
+                                         std::make_shared<StubFileSource>(),
                                          MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()));
 
     EXPECT_TRUE(map);