Auto upload: support USB mounted storage

The new auto upload utilizes the Media Scan implementation of Android. This works fine for internal memory and also SD cards mounted as internal storage. If the SD card is mounted as a USB storage the media scan doesn't seem to get triggered by the camera app but for some reason the Google Photos app will show the media folders on the storage point.
In order to be able to "just" query the USB storage folders a workaround might be that we simply trigger media scans of USB mounts from time to time which will add the media folders to the media database and thus makes them discoverable for us.

cc @tobiasKaminsky

This is an open issue for now and need some research on how this can be solved.