Fixes #122081.

When validating an Android Studio installation, add a warning validation message when we are unable to detect the version. This is because we have logic throughout the tool (JDK/JRE-searching) that is at higher risk of failing when we don't know the version.

Fixes #124756 by removing the concept of `Version.unknown`.

`Version` fields that needed the ability to represent an unknown version have been made nullable. Assigning `null` to them represents an unknown version.

This reverts commit 5fd259be.

* opt out the flutter tool

* oops EOF

* fix import

* Update tool_backend.dart

* Update daemon_client.dart

* fix more

In cases where the VSCode plugins are not located, display links to where they can be downloaded but do not surface an error.