Private DNS: Add user restriction

Add a user restriction to prevent the user from changing the Private DNS
settings.

This would be used together with the new DevicePolicyManager API to set
the Private DNS settings by the Device Policy Client to enforce Private
DNS settings by the IT admin of the user.

Bug: 112982691
Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testPrivateDnsPolicy
Test: m -j RunSettingsRoboTests ROBOTEST_FILTER=PrivateDnsModeDialogPreferenceTest
Test: Manual, using TestDPC
Change-Id: Ibec3cfcede7e4082db4068cbdd047ada3f6c227e