use Android 5.x and later API when available for downloads to SD card

[hmrct]

Background: beginning with KitKat, writing to the SD card by third-party applications was restricted. A new API allowing third-party apps to once again be able to write to the SD card was introduced in Lollipop (5.X).

Using this new API, when running on Android versions where it exists, will allow users to download large files directly to where they are more likely to fit, i.e., the SD card. Ideally, the new functionality would be transparent: a user would simply specify a directory on the SD card as the download directory, and it would "just work". As an example of how NOT to implement this feature, try using SFTP in the current-as-of-this-writing version of ES File Explorer app to copy a file to the SD card (on a Lollipop device). It works, but the user interaction required to enable writing to the SD card seems a bit awkward.

Thanks in advance for your favorable consideration of this feature request.