Add NetworkMonitor constants to SystemApi

NetworkMonitor lives in the NetworkStack which must only use system
APIs. This includes constants used by NetworkMonitor and captive portal
login.

Bug: 112869080
Test: m
(Cherry-pick of aosp I4879568d3fbf9435767c8d4d0ab5198d421a2f4f)

Change-Id: I8b5f0bb61c3ef0d2f33af7dd67b72ac6c5a1d1b5