DTD URI should be parsed using `--machine` output in `launch_app`

[gspencergoog]

Description

The DTD URI is currently parsed out of stdout, which doesn't work for Chrome targets because the --print-dtd flag doesn't print the URI on stdout for Chrome.

We should be parsing the flutter run --machine output for the DTD URI so that we can use a more reliable method of retrieving it, and because it does print the DTD even on Chrome targets.