personalization: set default to cbx screen saver

Set cbx screen saver as default when the users (who can see cbx screen
saver) turn on ambient mode.

BUG=b:284027348
TEST=ash_unittests --gtest_filter='*AmbientUiSettingsTest*'

Change-Id: Ibc6100013beb72968ec4bf228bce501b0c358f39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4583946
Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Thuong Phan <thuongphan@chromium.org>
Reviewed-by: Eric Sum <esum@google.com>
Cr-Commit-Position: refs/heads/main@{#1154603}