Make IdleDetector not emit resumed signal when it's inhibited

At the moment, when an IdleDetector is inhibited, it can emit the
resumed signal. It makes sense on one hand, but also it doesn't.
Inhibited != resumed.

According to the idle-inhibit-v1 protocol specification, we don't
need to emit the resumed signal:

> Likewise, the inhibitor isn't honored if the system was already idled at
> the time the inhibitor was established, although if the system later
> de-idles and re-idles the inhibitor will take effect.