Allow killed task to remove itself

When killing a task from the recents UI, the activities in
the task would be scheduled for destruction, and the task would
be forcefully removed from the ActivityStack, destroying the
ActivityStack in the process if this was the last task. Since the
ActivityStack was removed, any activities calling back to report
their onDestroy was called (or any timeouts that trigger if
activities fail to do this) would be dropped and certain cleanup
routines would not be called.

Tasks and their ActivityStacks are removed automatically when the
activities within them are removed, so this manual removal was
preventing the cleanup routines from running.

bug:10920157
Change-Id: Ied9d4f8fea761a373a9a80a0dfe810590ab411d8