The helper method in CtsNetUtils will not restore the private
dns hostname back if the device private dns mode is not strict
mode. It does not cause function break since the host
configuration is useful only when the private dns mode is in
strict mode. But tests should restore the setting back to its
old state to prevent break other tests or erase setttings.
Test: atest android.net.cts.ConnectivityManagerTest and
check if the private dns server configuration is
changed or not after testing
Change-Id: I7c85ddac7306c7c3eeac84679d96c4cfb11bd875
5e54074e6e