Creates an IntentService subclass for scanning removable "external
storage" for F-Droid package repos, e.g. SD Cards. This is intented to
support sharable package repos, so it ignores non-removable storage,
like the fake emulated sdcard from devices with only built-in storage.
This method will only ever allow for reading repos, never writing. It
also will not work for removeable storage devices plugged in via USB,
since do not show up as "External Storage"
* https://stackoverflow.com/a/40201333
* https://commonsware.com/blog/2017/11/14/storage-situation-external-storage.htmlcloses#1377
This uses the new Storage Access Framework, which was required for
accessing files on the SD Card starting in android-19. But the API
was really limited until android-21, and not really complete until
android-23 or even android-26. So the levels of usability will vary a
lot based on how new the version of Android is.
