Adds Semantic Widget to wrap the contents of each page's AppBar title attribute and adds header: true and headingLevel: 1 as Semantic attributes so the content of the title is compiled as an h1 tag to meet accessibility guidelines that each page must have a h1 tag present. Also updates the test cases for the home page and each use case page to check for its respective h1.
[Before Screenshot - Accessibility Assessments Home Page](https://screenshot.googleplex.com/4i9LuiGwvLnEcZ8)
[Before Screenshot - Checkbox List Title -- note: each use case page is missing an h1](https://screenshot.googleplex.com/3qQjfqvAMTehRsm)
[After Screenshot - Accessibility Assessments Home Page](https://screenshot.googleplex.com/APSJJXBmwNBP35m)
[After Screenshot - Checkbox List Title-- note: change is similar across each Accessibility use case page](https://screenshot.googleplex.com/6EGgZnTusEgeN5L)
Fixes b/338035526
As of https://github.com/flutter/engine/pull/47727 the web engine is able to find a default widget to focus on when a new route it pushed. The mobile engine already did that for some time. So `autofocus` is no longer necessary.
Mostly tweaks for better focus management, namely:
* Use `autofocus` throughout so the a11y focus is transferred to a logical place when overlaid content pops up (screen transitions, dialogs).
* Consolidate "enabled" and "disabled" widgets into the same screen. Otherwise, when only a disabled widget is shown, there's nothing to focus on and the screen reader is lost.
