[adaptive_scaffold] New and Improved Helper Widgets & Updated Examples and Their Tests

[gspencergoog]

This builds on https://github.com/flutter/packages/pull/2406, and is the work of @rena987, cloned from https://github.com/flutter/packages/pull/2439, I'm just landing it for her, since she has left.

Added builders and logic to the existing merged pull request above and updated examples and their respective tests.