Replace dns_tls with private_dns_mode setting

Also includes:
    - new layout fragment for private DNS configuration
    - PrivateDnsModeDialogPreference handler class

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net passes
    - manual interaction with private dns developer option works
Bug: 34953048
Bug: 6413396
Change-Id: I3a5490c21f7f3b3e523a247c287f213d19b75a8b