Prevent exception when stack being removed on crash

When an application has crashed, a request to finish activities
may remove some stacks, which was causing total number of display
children to be reduced and indexing error in a loop.
This CL removes optimization and always checks display child
count in the loop.

Bug: 76096678
Test: ActivityStackSupervisorTest#testRemovingStackOnAppCrash
Change-Id: I0d09dde9f5bef5a20f15dd06bddaccbd73679cdf