Add ExplicitHealthCheckService API

The ExplicitHealthCheckService implementation will live in ExtServices
so it can be updated outside of a platform update. It will contain
logic to assist the PackageWatchdog with observing packages, e.g
ensuring that the device can successfully connect to WiFi after a
mainline update.

Test: Builds
Bug: 120598832
Change-Id: If658567ca41bb0328e279735897fc50ab76abc11