[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/query.test.cpp

@@ -1,7 +1,6 @@
 #include <mbgl/test/map_adapter.hpp>
 
 #include <mbgl/map/map_options.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/test/stub_file_source.hpp>
 #include <mbgl/test/util.hpp>
 #include <mbgl/util/image.hpp>
@@ -35,9 +34,8 @@ public:
 
     util::RunLoop loop;
     std::shared_ptr<StubFileSource> fileSource = std::make_shared<StubFileSource>();
-    ThreadPool threadPool { 4 };
-    HeadlessFrontend frontend { 1, threadPool };
-    MapAdapter map { frontend, MapObserver::nullObserver(), fileSource, threadPool,
+    HeadlessFrontend frontend { 1 };
+    MapAdapter map { frontend, MapObserver::nullObserver(), fileSource,
                   MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize())};
 };