Adds usage of SystemChrome.setAPplicationSwitcherDescription to update the page title for the AutoComplete demo page.
[Before screenshot: example AutoComplete](https://screenshot.googleplex.com/8DpESj4QrQA2YrW)
[After screenshot: example AutoComplete](https://screenshot.googleplex.com/6AdfSA8a8VwUaXM)
Fixes b/338056490
*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*
*List which issues are fixed by this PR. You must list at least one issue. An issue is not required if the PR fixes something trivial like a typo.*
*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*
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
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.
