Don't start a dimming operation when hiding.

Check to see if app token is hiding before going ahead with turning on
dimming. Before this fix went in we were turning dimming back on right
after turning it off. Then we didn't turn it off again until all
animations had completed leading to a delayed dim-off experience.

Fixes bug 6378033.

Change-Id: Ic819a0093ba95f62df369266c07525835703c5fa