Avoid requesting stop for finishing activity

The problem is that removeTask is called on a task with an activity
which is finishing and stopping. Because it is already finishing,
the steps of removeTask are skipped. If the activity is removed
from mStoppingActivities, then the activity won't proceed to destroy.

Bug: 309136273
Test: atest ActivityTaskSupervisorTests#testRemoveTask
Test: CtsWindowManagerDeviceActivity:ActivityLifecycleTests \
      #testRelaunchConfigurationChangedWhileBecomingVisible
      There should have wm_on_destroy_called for SecondActivity.

Change-Id: If13d893ae7eb4cc4105f5554a6e3ed6aae728df7