refactor: don't use Cronet directly for downloading

[Bnyro]

• this allows to remove Cronet as per [meta] drop cronet #6867 or make it optional, depending on the state of the discussion
• apart from this, the refactor here has been needed for a long time to make the download service more comprehensive
• using OkHttp instead of a HttpUrlConnection instance directly allows us to act on a higher level, the HTTP client might also fix some issues experienced by users, for instance it automatically retries to connect if the connection fails

possibly resolves #5661