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
(Cherry-pick of aosp/890004)

Change-Id: I4bccfbd87bae5b2d65e45c7a5918aa45ab5d76e8