Captive portal: fix probe urls at network creation

This patch changes how url configuration values for captive portal
detection are read: instead of re-reading the settings at every probe
detection, the settings are read once when the NetworkMonitor associated
to a network is created.

If the settings are updated, the new values are picked up by new
networks, but not by existing networks. Since captive portal detection
is most important when joining WiFi networks, the newest available
settings values will still be used.

This change prepares introducing further changes for rotating probe
urls. Especially it helps making the detection strategy stateful with
respect to the lifecycle of a single NetworkMonitor.

Test: built, flashed, tested manually with various portal networks
Bug: 36532213
Change-Id: I71cc8bb8b996462f27f50798d67bceee5ffb898d