New Settings symbols for captive portal detection

This patch defines new Settings symbols for
  - setting the probe urls for captive portal detection.
  - setting which User-Agent to use for captive portal detection.

The existing default values for these settings are not changed, i.e:
  - HTTP and HTTPS probes urls are unchanged.
  - the fallback probe is not used.
  - User-Agent is empty by default.

Bug: 29367974
Change-Id: I6e4b3b172e56b8b67fffa4b51f776d68d5851f25