Change network validation probe default configs

This patch changes the default configuration values for captive
portal detection.

Now by default:
  - the HTTPS probe tries to connect to www.google.com/generate_204
    instead of connectivitycheck.gstatic.com/generate_204
  - the fallback probe is used and tries to connect to
    google.com/gen_204
  - the request header User-Agent field is set to appears similar to
    a Chrome request.

Bug: 29367974
Change-Id: Ic965e2693e154e96bf0f7e4c1b9a2f704b9c3452