CrOS Settings: Fix a11y label and description for settings-dropdown-v2

Also ensures the a11y label and optional a11y description is not
duplicated when being read by screen readers.
Screenshot: http://screen/6DVF7LrU3CxTcqH.png

Bug: b:333454097
Test: browser_tests --gtest_filter="*OSSettings*SettingsDropdownV2*"
Change-Id: I3ab7487d719fefd6479ec3261eee592deecaba4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5607254


Reviewed-by: Connie Xu <conniekxu@chromium.org>
Commit-Queue: Wes Okuhara <wesokuhara@google.com>
Cr-Commit-Position: refs/heads/main@{#1312335}