Temporarily restore the net.dnsX system properties.

This will limit app breakage in dogfood. The plan is to remove
these obsolete system properties again in O developer preview
builds so that application developers can move away from them.

Test: marlin boots and net.dnsX exists and is updated correctly

Bug: 33308258
Bug: 33807046
Bug: 34028616
Bug: 34115651
Change-Id: Iaf2fa213c314b7ba251c065b304e7e9869bafe9f