Use getPrivateDnsMode() to get the value of private DNS

PRIVATE_DNS_DEFAULT_MODE_FALLBACK has removed, use
ConnectivityManager#getPrivateDnsMode() to get the value of
private DNS instead of querying settings and implementing logic
by itself.

Bug: 172183305
Test: make RunSettingsRoboTests \
      ROBOTEST_FILTER=PrivateDnsPreferenceControllerTest
Test: make RunSettingsRoboTests \
      ROBOTEST_FILTER=PrivateDnsModeDialogPreferenceTest
Change-Id: I024c3ddfb85f4c3d177e2e99294c998d81d4e08d