Show a screen unavailable message for disabled screens.

[kenzieschmoll]

When a first party DevTools screen is loaded but is not available for the current app, we now show a message explaining why. Before we just silently navigated to a different screen or showed a blank page when embedded.

Now, we show this:

Embedded mode (hide the navigate home button in this case):

Fixes https://github.com/flutter/devtools/issues/7879