Add private dns dialog in wifi settings

This option lives in menu. A dialog will show up once this menu
is clicked.

We reuse most of the logic in go/aog/524415 with the following
updates:
1. Use radioGroup instead of directly control radioButton
2. remove bunch of methods if they are only used once.

Following cl will:
1. Remove this feature in development page
2. Add the help link at the bottom of the dialog
3. Disable "Save" button when hostname is invalid.

Bug: 68030013
Test: add for later

Change-Id: I4c6d359dc9c55675858c20e47953ef677b31c3b5