Avoid double call to set ofono's online property

On initialization a double call to set-soft was being done for the ofono device. This caused bugs LP #1392397 and #1379807.
Move all these additions (the g_cancellable_cancel() call and signal emission, if absolutely necessary) to the dispose function. You really then only should need the g_cancellable_cancel, since when you're unref'ing the modem you've already removed it from the arbitrator beforehand, which further reduces the likelihood of callbacks having to fire.