SubDownload: keep file in app storage

With some file permissions, we might be able to read a file from a specific directory but not write to it. Ex: video permission in Android 14, won't write to a directory "samples" at the root of the storage. This causes the downloadSuccessful's copyFile to fail and throw an exception because it can't write, and the downloading animation is never cancelled

This is, I think, why we had some user reports saying that they couldn't download subtitles, even after the rework. It seems to fix the issue, but I'm opened to suggestions, I have the feeling I'm missing something ...