ScreenShield: tie the suspend inhibitor to our isActive property

ScreenShield: tie the suspend inhibitor to our isActive property

The whole point of holding a suspend inhibitor is to be able to lock
before suspending.

Currently, when resuming we immediately take the inhibitor without
checking that we're locked which means that we won't be able to
release this inhibitor if we don't unlock at least once.

To prevent that and to better match the inhibitor's intention in the
first place, we can tie the inhibitor with not being locked. In
practice, we also want to let the locking animation finish before
suspending, so we'll tie the inhibitor with not being active
instead.

https://bugzilla.gnome.org/show_bug.cgi?id=749228