[ios, osx] Unified offline and ambient caches

There is now only one instance of mbgl::OfflineFileSource, created when the shared MGLOfflineStorage object is initialized. Also create and use the shared MGLOfflineStorage object when initializing an MGLMapView object. The unified cache file is located in a subdirectory of Application Support, where the SDK has control over the file’s lifetime. The subdirectory is already named after the host application’s bundle identifier, ensuring that each Mapbox-powered application has an independent tile limit. If there’s an ambient cache in a subdirectory of Caches, delete it. If there’s an offline cache in a subdirectory of Documents on iOS or Caches on OS X, move it to the unified cache location in a subdirectory of Application Support.

Fixes the iOS/OS X side of #4338.

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