ClatCoordinator: improve file descriptor clean up in error handling

- Make the file descriptor clean up better when an error happens.
- Remove unused TODO for iptables because it has been
  implemented by another approach. See b/218407445.

Test: atest com.android.server.connectivity.ClatCoordinatorTest
Change-Id: Ib8bfa8875ae774769ab7e416fa7e5fde140f58da
2 files changed