[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/native_map_view.cpp

@@ -21,7 +21,6 @@
 #include <mbgl/util/exception.hpp>
 #include <mbgl/util/geo.hpp>
 #include <mbgl/util/image.hpp>
-#include <mbgl/util/shared_thread_pool.hpp>
 #include <mbgl/util/logging.hpp>
 #include <mbgl/util/platform.hpp>
 #include <mbgl/util/projection.hpp>
@@ -66,8 +65,7 @@ NativeMapView::NativeMapView(jni::JNIEnv& _env,
                              jni::jboolean _crossSourceCollisions)
     : javaPeer(_env, _obj)
     , mapRenderer(MapRenderer::getNativePeer(_env, jMapRenderer))
-    , pixelRatio(_pixelRatio)
-    , threadPool(sharedThreadPool()) {
+    , pixelRatio(_pixelRatio) {
 
     // Get a reference to the JavaVM for callbacks
     if (_env.GetJavaVM(&vm) < 0) {
@@ -89,7 +87,7 @@ NativeMapView::NativeMapView(jni::JNIEnv& _env,
 
     // Create the core map
     map = std::make_unique<mbgl::Map>(
-        *rendererFrontend, *this, *threadPool, options,
+        *rendererFrontend, *this, options,
         mbgl::android::FileSource::getSharedResourceOptions(_env, jFileSource));
 }