Keep a copy of each engine's description that can be accessed outside the engine's UI thread (#6885)

The service protocol's ListViews method needs to return description data for
each engine in the process.  Previously ListViews would queue a task to each
UI thread to gather this data.  However, the UI thread might be blocked from
executing tasks (e.g. if the Dart isolate is paused), resulting in a deadlock.

This change provides a copy of the engine's description data to the
ServiceProtocol's global list of engines, allowing ListViews to run without
accessing any UI threads.

Fixes https://github.com/flutter/flutter/issues/24400