Enterprise Policy for Private DNS Setting

A new API for setting the Private DNS settings value programatically via
the DevicePolicyManager.

Since there are two separate settings for Private DNS, and the value
provided for the hostname needs to be validated, a new
DevicePolicyManager API is introduced.

Only a Device Policy Client in Device Owner mode may change these
settings.
The DPC may additionally set a user restriction (added in a separate CL)
to prevent the user from changing Private DNS settings.

Bug: 112982691
Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testPrivateDnsPolicy
Change-Id: I566437e4fe10e1346858149120c50b3c20ca073f