Add help text to "add new VPN" dialog and server_address preference

Show a couple of hostname examples so the user knows the expected format.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
AuthFormHandler: Add padding to make the dialog less ugly

These constants were borrowed from AOSP's alert_dialog.xml.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
screenshots: Update to match recent UI changes

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>