Allow downloading to SD card by moving files afterwards

[devgianlu]

An user suggested:

In that case, I think you could use a private directory on sdcard (sdcard/Android/package name) as a temporary directory, which is still writable without any extra permissions other than storage access, and then move a downloaded file using storage access framework to the directory that the user picked on the external storage. Since your app already supports storage access framework, this should not be too hard to implement.