Updated media adding function and media tests.

Media add now correctly normalizes file names.
Media tests no longer clear the primary collection.media folder on a device.

Also removed the public getCurrentAnkiDroidMediaDir() method because:
1 - It makes an assumption about the collection name (it's wrong)
2 - You can get it from the media manager if you need it
3 - You shouldn't need to access it as all media operations should be done through the media manager anyway.
Merge pull request #466 from hssm/media-dev

Updated media adding function and media tests.