Unify offline and ambient caches

[1ec5]

Now there’s 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.

There are a couple consequences to this approach:

1. On iOS, it isn’t possible for a user to export their offline cache to another device or import it from another device. The application would have to move the file into Documents first.
2. If disk space runs low on the iOS device, the unified cache will continue to eat up disk space, potentially due to ambiently cached content, and the only way to reclaim that space would be to delete the application (along with all the offline content). See Add API to clear ambient cache #4376 for a possible solution.
3. On OS X, it isn’t possible for two Mapbox-powered applications to share an ambient cache. The unified cache’s path must include the application’s bundle identifier, because otherwise one application’s offline packs would count against another application’s limits.

Fixes the iOS/OS X side of #4362.

/cc @jfirebaugh @boundsj

[jfirebaugh]

👍

I got tripped up by simulator behavior, but after figuring that out, I can confirm that the migration works and offline packs are used when offline.

[1ec5]

Builds are passing; merging now.