Add support for assertive announcements in aria-live

[nbayati]

Add support for two different modes of aria announcements: polite/assertive via setting the aria-live flag.
This PR makes the default to be assertive and provides an optional flag to set the mode to polite
Fixes flutter/flutter#104109

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[yjbanov]

Only a few nitpicks in this PR. This overall LGTM modulo the ariaLivePolitenessSetting name in the protocol, which sounds too much web-specific. I think we should keep the framework-side as cross-platform as possible.