`StopOrRestartTracking` dbus method triggers multiple `FactsChanged` signals when restarting an activity

[johnnyg]

What

StopOrRestartTracking causes two FactsChanged signals

Steps to reproduce

1. Subscribe to the FactsChanged signal
2. Start an activity
3. Stop the activity
4. Call the dbus method StopOrRestartTracking

Expected behaviour

you receive a single FactsChanged signal

Actual behaviour

you receive two FactsChanged signals

Details

It seems that the dbus API calls self.facts_changed() regardless of whether it stopped or restarted the activity however when we restart the activity we call self.add_fact(...) which also calls self.facts_changed().