Listen to exposure change in addition to focus change

Focussing only works when using the rear-facing camera. However, changing the exposure works with both cameras (front- and rear-facing). By listening when the exposure gets changed, we can take advantage of animating `_focusView` when using the front-facing camera. Hoorah for no longer needing to hide `_focusButton` when using the front-facing camera (simple == better)!