Add a boolean flag to show or hide Private DNS in Network & internet.

The following boolean flag is added:
config_show_private_dns_settings

When set to false, Private DNS will not be shown in Network & internet.

Bug: 111398942
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=PrivateDnsPreferenceControllerTest
Change-Id: I62911930680355b7fc109b295d8526bdb7eac0b3