fix(Dropdown): aria-live region needs aria-atomic

[fracmak]

Did an audit of our site with NVDA and ran into a problem where dropdowns act kind of funny depending on the order of the values in the dropdown. Apparently aria-live regions only read what changes in the region unless you specify aria-atomic to read the entire area aloud. This means that if you have a dropdown like this

Within 25 miles
Within 50 miles
Within 125 miles
Within 250 miles
Within 500 miles
More than 500 miles

The screen reader reads these options as 'Within 25 miles', '50', '125', '0'

https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Live_Regions#Advanced_live_regions

[codecov-io]

Codecov Report

Merging #3621 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3621   +/-   ##
=======================================
  Coverage   99.81%   99.81%           
=======================================
  Files         174      174           
  Lines        2744     2744           
=======================================
  Hits         2739     2739           
  Misses          5        5

Impacted Files	Coverage Δ
src/modules/Dropdown/Dropdown.js	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e595719...8fc28ad. Read the comment docs.