OfflineManager and MapView should share a singleton DefaultFileSource

[1ec5]

As of #4383, all instances of mbgl::DefaultFileSource in the Android SDK are backed by the same file on disk. Per #4362, the singleton OfflineManager and all mbgl::View instances managed by MapViews should ideally share the same instance of mbgl::DefaultFileSource, so that the 20-connection throttle can be applied to the whole process instead of on a map-by-map basis.

In the iOS and OS X SDKs, in #4377, we made the singleton MGLOfflineStorage object responsible for the singleton mbgl::DefaultFileSource instance. Thus MGLMapView depends on MGLOfflineStorage (as ambient caching is just a special case of offline caching), but not the other way around.

/cc @zugaldia @jfirebaugh

[tobrun]

This landed as part of #8083