DebugConnection should not expose internal dwds vmService

Since the addition of DDS we don't want the users of dwds to have access to the internal dwds vmService, as it will lead to bypassing DDS.

See example where devtools couldn't check flutter version and as a result could not enable layout explorer:

https://github.com/flutter/devtools/issues/2832

We could remove the vmService from `DebugConnection` API so the clients can only access the VM via a websocket, as it is done in the following fix for the issue above:

https://github.com/flutter/flutter/pull/79479

Alternatively, we could expose some connector to the VM that hides what is actually running under the hood.

Start by locating the DebugConnection API and tracing how its internal dwds vmService is exposed. Review the linked DevTools issue and Flutter pull request, then determine whether the API should remove vmService or provide a connector; done means clients can access the VM without bypassing DDS.

Written by the indexing model from the issue text.