Use [set|get]PrivateDnsMode directly

The value of PRIVATE_DNS_MODE_* has changed from String to int and
ConnectivityManager provides 2 new APIs to set and get private
DNS mode, the caller can use them directly and doesn't need to update
Settings by itself.

Bug: 185311744
Test: 1. make RunSettingsRoboTests ROBOTEST_FILTER=PrivateDnsModeDialogPreferenceTest
      2. make RunSettingsRoboTests ROBOTEST_FILTER=PrivateDnsPreferenceControllerTest
Change-Id: I47706efe9c53a00438fdec97fda244638cecb3dc