downloadMedia
Downloads all available media items in full resolution, so that all items currently available as thumbnails (or only as filenames) will be stored locally and made accessible as localMedia.
This is the equivalent of calling turnOnMediaDownload, waiting for all downloads to complete, and call turnOffMediaDownload again.
Scheduler: The returned Completable will operate on the IO Scheduler.
Return
A progress indication of the overall download progress of the media items. See MediaItemDownloadProgress in more details of the provided progress.