Dismiss exclusion region when task is hidden.

Previously we only dismissed gesture exclusion regions when a decor was
closed. This meant the regions persisted when hiding the task or exiting
desktop windowing, interfering with back gestures.

Fix: 368697694
Test: Manual; line up 4 app headers on the left edge of the screen. Exit
desktop and confirm back gesture still works in the previously occupied
region.
Flag: EXEMPT, bug fix

Change-Id: I27afd66108121986bbe53e5d8cf5afdb1d506627