Restore the PrivateDnsMode emun to its original definition

Westworld requires the first field of all enums to be set to
UNKNOWN/UNSET, but this is awkward for the resolver code and
makes it hard to verify whether we have any PrivateDnsMode
variables initialized to UNKNOWN by accident.

So revert this change, and add a simple function to convert
the internal PrivateDnsMode enum to the other PrivateDnsMode
emun defined in the stats proto.

Bug: 119862317
Test: atest
Change-Id: Id146a16e41e0183867b3a4e2d87a2b0fc09df27d
Merged-In: I35908fac7f089c1fac507bae63b02ebb46ad50a9
Merged-In: I6054faa82f9a3cdc843fd6dcbd11d5cef75edd98
Merged-In: I74c7b4145d1469010d3e205183012a9b830f0a55
Merged-In: Id146a16e41e0183867b3a4e2d87a2b0fc09df27d