Clean up expired DnsTlsDispatcher::Transport if DoT server list is empty am: 23d7371b

Original change: https://android-review.googlesource.com/c/platform/packages/modules/DnsResolver/+/2096574



Change-Id: If139787faa9ed5adba1eba94fdfcf0de79d984c2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>