Add CaptivePortalData fetching to NetworkMonitor

When a capport API URL is sent through notifyNetworkConnected or
notifyLinkPropertiesChanged, NetworkMonitor will (re)evaluate to
fetch the data, and report it back via notifyCaptivePortalDataChanged.

Test: atest NetworkStackTests NetworkStackNextTests
Bug: 139269711
Change-Id: I101e2b2799d399fa7b560bf5ea3d86dac7ba1be9