Add SystemApi for captive portal metrics

The metrics go through NetworkMonitor in the NetworkStack so that they
can be upgraded to new metrics in the future.

Test: flashed, captive portal login works, metrics shown in events log
Bug: 112869080
Merged-In: I4bccfbd87bae5b2d65e45c7a5918aa45ab5d76e8
Change-Id: Ib5e2126788f8d56a00a56d7efcd33c5f9a37a6de