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

Screenshot: https://screenshot.googleplex.com/78VZSwVfkxcpr37

Bug: b/345837068
Test: browser_tests --gtest_filter="*OSSettings*SettingsSliderV2*"
Change-Id: I262e78ff11c2210863a2d9ffc7b0f850bbd6236e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5618010


Reviewed-by: Wes Okuhara <wesokuhara@google.com>
Commit-Queue: Connie Xu <conniekxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313166}