Possibility to cancel downloading of offline pack.

[dMozz]

Platform: iOS
Mapbox SDK version: latest version

Steps to trigger behavior

1. Start downloading of offline pack
2. Try to suspend or remove downloading pack.

Expected behavior

Pack downloading stop and downloading process remove from downloading queue.

Actual behavior

Downloading continuous. If at that point I try to open another screen with map I see black view. Map appears only when pack downloading finished.

[boundsj]

This is tracked in #5538

[boundsj]

Actually, @1ec5 dropping notifications (#5538 (comment)) will appear to allow canceling a download but it probably won't solve the black map issue noted here, will it?

[friedbunny]

Offline downloading blocking maps from being displayed is over in #4414.