Private DNS: Fix handling of null mode

Address the case where the Private DNS mode has not been set at all, so
the value obtained from settings when reading it is null.

Java cannot cope with null value in a switch statement, so translate
that case to the default value (from the ConnectivityManager), in the
same way the Settings code does when presenting Private DNS mode to the
user.

Bug: 112982691
Test: Factory reset a device, then atest com.android.cts.devicepolicy.DeviceOwnerTest#testPrivateDnsPolicy
Change-Id: Ife6b5c15b70517f75d8815103a19a9ef72122212