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
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