Fixed leak of app token in exiting list.

- Remove app token from exiting list when it's last window is removed.
- Remove app token from exiting list when it's removal isn't delayed.

Test: Launch a few apps and press back and make sure the exit app to
list dump is empty.
Fixes: 38337456

Change-Id: Ia2f22c6f1add5f867e25583ec38ffccc71217ee7