Support dynamic a11y strings for i18n changes

[zepumph]

Promoting to its own issue from #499. Basically there were many spots in the PDOM, voicing, and describers implementations that we totally punted back in that issue. Which is fine! All fine. But when we/if we revisit this sim, it would be worth discussing how much we want the a11y strings to be able to change dynamically as well from stringProperty changes. See all the TODOs pointing to this issue!