Check if WindowToken is visible before adding dim.

If a window token is set to hidden, we don't want to add dim layers to any
window states for that window token.

For example, if an AWT is set to exit, but one of its windows has a dim,
we don't want the dim to stay while the app is exiting.

Fixes: 72821785
Test: Press home from app with dialog open. Dim should hide immediately
and not wait until exit animation is complete.
Test: atest WindowManagerSmokeTest

Change-Id: I0f87dd3900fc4535f572cf2b876e29d9b27f0b35