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
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*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*
