Prevent windows from dimming if host is going away

The exit animation has already started, and if someone keeps dimming,
then it would
- create a new surface (because exit animation removes the dim state)
- see that we need to interrupt the dim, since host is leaving
- destroy the surface
- repeat

Bug: 401449608
Fix: 403209573
Test: DimmerTests
Flag: EXEMPT minor change
Change-Id: I6bb6c540aba3d473e1587b4a7e0bef1883c335cd