Private DNS: Add policy transparency

When the Private DNS setting is disabled via a user restriction, show
information to the user explaining that they cannot change this setting
due to IT admin policy.

Testing steps:
* Installed & configured TestDPC on the device.
* Made sure that the Private DNS setting is usable.
* Set the user restriction via TestDPC ("User Restrictions" -> "Disallow
config Private DNS").
* Navigated to Settings -> Network & Internet -> Advanced
* Observed that Private DNS is greyed out and the Restricted icon is
showing.
* Observed that when tapping either the icon or the greyed-out text I
get a dialog with policy information.

Implementation notes:
I have not utilized the existing RestrictedPreference as the Private DNS
mode preference extends CustomDialogPreferenceCompat.
I have also not utilized the RestrictedPreferenceHelper as it modifies
the summary shown for the preference. This preference has its own
summary (showing the current state of Private DNS configuration) which I
did not want to override.

Bug: 112982691
Test: Manual, see above.
Change-Id: I9b7496b5b2cbb26012d889369f2199239cd2e00f