Hide private DNS settings UI in Guest mode

Hide private DNS settings UI in Guest mode to prevent guest
users modifying global private DNS settings.

Bug: 206987762
Test: 1. make RunSettingsRoboTests \
         ROBOTEST_FILTER=PrivateDnsPreferenceControllerTest
      2. Switch to Guest user and check if the private DNS UI is
         hidden or not.
Change-Id: Iebfb8684da3be32110decd9e8447dd07b1c40387
(cherry picked from commit 52e863b5)
Merged-In: Iebfb8684da3be32110decd9e8447dd07b1c40387