Clean up expired DnsTlsDispatcher::Transport if DoT server list is empty

If we get no usable servers from getOrderedAndUsableServerList(),
we need to check and remove expired Transports; otherwise, we won't
be able to remove expired Transports if all the Transports are marked
as unusable.

Bug: 79727473
Test: atest
Change-Id: Ib04c0635e7e2a43bfd532d086d1f7acc8962392f