If658567ca41bb0328e279735897fc50ab76abc11 added the
ExplicitHealthCheckService API to allow ExtServices to request explicit
health checks for packages and report to the PackageWatchdog.
We now implement this service and add a rudimentary NetworkChecker
that relies on listening to the ConnectivityManager for validated
network requests.
Bug: 120598832
Test: Manually tested that after an APEX update, the network stack
does not pass the explicit health check until WiFi is connected
successfully. If Wi-Fi is never connected and the network stack
monitoring duration is exceeded, the update is rolled back.
Change-Id: Ifa968b4c7107cabedf186dffbc5eb771a3840e7f
e680d60f32