Display Private DNS status in preference summary

Also: fix a bug where the actual current mode is not read correctly
from settings on initialization.

Test: as follows
    - built, flashed, booted
    - manual test behaves as expected
    - make -j50 RunSettingsRoboTests ROBOTEST_FILTER=PrivateDnsModeDialogPreferenceTest passes
    - make -j50 RunSettingsRoboTests ROBOTEST_FILTER=PrivateDnsPreferenceControllerTest passes
Bug: 34953048
Bug: 64133961
Change-Id: I0b845b3894a47b837abf5de273d1ada642ef5a23