Support inspecting widget tree of multi engine

Currently, when using flutter attach, the widget tree in Flutter Inspector page only shows first engine's widget tree, we can't choose to see widget tree of other engine.

Start by reproducing the issue with `flutter attach` and opening the Flutter Inspector page for an application with multiple engines. Trace how the inspector currently selects and displays the first engine's widget tree. Done means the inspector lets users choose another engine and displays that engine's widget tree.

Written by the indexing model from the issue text.