- 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`.
Before this change, ImageManger's 'done' callback for onStyleImageMissing observer
notification that was created on renderer thread, could be called from different thread,
therefore, is not thread safe. For example, on Android platform, callback was invoked
from UI thread. This change makes callback to be scheduled on originating thread.
