CrOS Settings: Define SettingsSliderV2, including pref logic, unit tests

Use PrefControlMixinInternal for SettingsSliderV2 which allows clients
to use it both with and without a pref specified.

This CL also includes the logic for showing / hiding the managed icon.

Bug: b/333453944, b/333454182
Test: browser_tests --gtest_filter="*OSSettings*SettingsSliderV2"
Change-Id: Ic3f686bfea0c16215d74da036e4b3d0e13578a64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5468882
Reviewed-by: Wes Okuhara <wesokuhara@google.com>
Commit-Queue: Connie Xu <conniekxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1292626}