Before this change, there was logic in `flutter attach` that would try
and listen for a new application to attach to in the scenario the
current target application disappeared. This behavior isn't documented,
and has resulted in a `StateError` being thrown due to the VM service
URIs stream being listened to multiple times.
This change removes this behavior, which also prevents the `StateError`
from being thrown.
Fixes https://github.com/flutter/flutter/issues/156692, which is a
top-10 crasher for the tool.
4c1b7f8e6d