Separate calls for media file types

Whilst looking into an issue with b2gpopulate I found a couple of issues with gaiatest. First, we currently get **all** media types in one go. By separating these they all have a fresh timeout and can fail independently. It also means we could later assert file counts for each type, and allows us to extend to support more types.

I also found that we were overriding the script timeout when we restart B2G, and were unnecessarily retrieving all media files twice.