Calling pause for the discovery should cancel the current folder discover

When the medialibrary is discovering, parsing a folder (especially a large folder on a slow SMB share) can take some time.

As the media parsing is locking further calls to it, it locks VLC Android's app browsing if the user is trying to browse a local folder while the ML is stuck on a large folder.

The solution would be to pause the discover when the user starts browsing but we have to make sure that the current operation done by the Ml is canceled at this time to let the app be able to start browsing.