Use filtered list for ext storage list mounts API

Now the external storage correctly returns the mount points visible only
for the current user by using the method getAbsoluteMountPoints() which
is already filtered.

Since that call was missing the backend name which is important for the
UI, this one was added as well.

Fixes https://github.com/owncloud/core/issues/9279

Please review @icewind1991 @DeepDiver1975 @Xenopathic