Stop setting net.dns{1234} system properties.

Nothing in the system consumes these properties, they were never part
of an official public API, and they can leak information about the
current network to applications without networking prvileges.

Specifically, these properites allow applications to access network state
(and detect network changes, etc.) without having the ACCESS_NETWORK_STATE
permission.

Test: runtest ConnectivityServiceTest passes
Bug: 33308258
Change-Id: I85abc52bc11aee33940fd0af2397478a9a5c3b8f