rhbz#1016758: Dispose bridges when disposing bridge factory

... (which happens when the service manager is disposed), so that no bridges can
still run during exit.  But this is so glaring that I wonder whether I missed
the obvious when I originally wrote that code, or whether I rather miss the
obvious now.  So better let this rest for a while on master before deciding
about any backports.

Change-Id: I7c9ad6c8a53dfd1a7b702640920dcb0a9a2c3007