Migrate existing usages of embed=true query parameter for DevTools screens to embedMode=one

[kenzieschmoll]

As of flutter/devtools#7710, which was rolled into the Dart SDK in https://dart-review.googlesource.com/c/sdk/+/365620, DevTools expects a new format for setting embedded mode. We will still need to use the legacy query parameter for Dart SDK versions before https://dart-review.googlesource.com/c/sdk/+/365620.

There are now two embedded modes that can be set through a query parameter: embedMode=one (for embedding a single screen, which is the current behavior) or embedMode=many (for embedding many screens, which is newly supported behavior added for #7195).

The change was made to be backwards compatible with embed=true, but we should migrate to the new format so that we can remove the legacy handlers.