Additional cleanup after stack deletion.

- Remove activity from PendingActivityLaunch list when it is removed
from stack. This prevents the delayed launch causing
IllegalArgumentException in b/16045752.
- Move PendingActivityLaunch from ActivityManagerService to
ActivityStackSupervisor.
- Immediately call onTaskListEmptyLocked() in cases where no
activities are found in stack.

Fixes bug 16045752.

Change-Id: Ia69a449e7f5e08ab6e36157d0fd793c4d2fdaca4