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