Restore the net.dnsX system properties.

In order to avoid app breakage, we are putting back the system
properties and using selinux to disallow access from apps
targeting the O SDK or above.

This CL is cherry-picked from internal commit
e3b85dfb, but with a different
commit description.

Bug: 33308258
Bug: 33807046
Bug: 34028616
Bug: 34115651
Test: system properties are readable as root and shell
Change-Id: I8d51e8e0a620d581c4251fb2f3c1d0a813b929be