DTD only supports a single instance of a registered service with a given
name. For widget preview development, we sometimes want to use a DTD
instance that's attached to an IDE to test IDE integration. However,
IDEs frequently spawn their own widget preview instances which register
services with DTD. In the case where `flutter widget-preview start
--dtd-url=<dtd-url>` is run and `dtd-url` points to a DTD instance with
another widget preview service running, the process simply crashes.
This change adds a unique identifier to the widget preview DTD service
and stream names that allows for each `flutter widget-preview start`
instance to register its own unique widget preview DTD services, even if
other widget preview instances are using the same DTD instance.
Fixes https://github.com/flutter/flutter/issues/179883
afb13d8bb7