Consider Private DNS user restriction

As a new user restriction, to disallow changing Private DNS settings, was
added, make the Private DNS mode dialog preference consider whether to
let the user modify it or not based on the presence of this restriction.

Bug: 112982691
Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testPrivateDnsPolicy
Test: m -j RunSettingsRoboTests ROBOTEST_FILTER=PrivateDnsPreferenceControllerTest
Test: Manual, using TestDPC

Change-Id: If815860ace3aadf6f79fd23173f0a2c80a29f0e1