File field on App Size screen should be editable

While working on https://github.com/flutter/devtools/issues/2209, I discovered that I could not type into or copy and paste into the file field on the App Size screen. Considering there is a message that displays the path to the file that is generated when running the `flutter build apk --analyze-size`, it would be nice to be able to copy and paste the path into the file field rather than have to navigate to the file.