Change the parameter type from ContentResolver to Context

Context is more useful than ContentResolver, it can provide more
information if we want to change the behavior in the future.

Bug: 172183305
Test: make RunSettingsRoboTests \
      ROBOTEST_FILTER=PrivateDnsPreferenceControllerTest
Test: make RunSettingsRoboTests \
      ROBOTEST_FILTER=PrivateDnsModeDialogPreferenceTest
Change-Id: I6c06460b67f81112db311f0739457d5f1e2e4554