[iOS] MGLOfflinePack downloads do not continue in the background

[felixLam]

Downloads of MGLOfflinePack do not continue while the app is in the background. It would be nice if there was a way to specify the url session to use for the download in order to use a backgroundSession which could continue in the background.

Platform: iOS
Mapbox SDK version: 3.6.3

Steps to trigger behavior

1. Create a new MGLOfflinePack or resume an incomplete one
2. Leave the app while the pack is downloading
3. Wait a while
4. Open the app back up

Expected behavior

While the app was in the background the downloads continue and the progress changed.

Actual behavior

The downloads where paused while in the background and the progress is unchanged.

[jfirebaugh]

Duplicate of #4291.