Factor in PRIVATE_DNS_MODE_DEFAULTSETTING value

This setting can be used to change the in-code default.

Test: as follows
    - built, flashed, booted
    - make -j50 RunSettingsRoboTests ROBOTEST_FILTER=PrivateDnsModeDialogPreferenceTest passes
    - make -j50 RunSettingsRoboTests ROBOTEST_FILTER=PrivateDnsPreferenceControllerTest passes
Bug: 79719289
Change-Id: I27e5fb9cc66dde1df72395c22179bca3f62ef782
4 files changed