Create a string for private DNS "On"

The last private DNS settings screen landed in P after string
freeze, which led us to reuse R.string.switch_on_text for the
"on" text when private DNS is active. That string comes from
notifications. Private DNS should have its own string for this
instead.

Bug: 79122154
Test: 1. Build pass
      2. make -j44 RunSettingsRoboTests \
      ROBOTEST_FILTER=PrivateDnsPreferenceControllerTest

Change-Id: Ie013a858c8bc41e00a1b940d02efa2b605991685