extensions-app: Start as service when D-Bus activated

When the app is properly launched, D-Bus activation will be followed by a call to org.freedesktop.Application.Activate() to open the window.

But it's also possible for D-Bus activation to happen in other circumstances, for example during gdbus command line completion.

We don't want to pop up a window then, so pass the --gapplication-service flag to properly separate D-Bus activation from activating the app.