mpris: Force refetching of Position property

Position is not supposed to be notified, so for some clients
GDBus does not update the cache although it changes.

We invalidate the cache here so that it will always be re-fetched
from the peer.

Signed-off-by: Jens Georg <mail@jensge.org>
mpris,external: Fix plugin activation

Signed-off-by: Jens Georg <mail@jensge.org>