Add ExplicitHealthCheckController

This controller will handle all explicit health checks to
packages for the PackageWatchdog. It has the following
responsibilities:
1. Keeping the required packages alive so they can respond to explicit
health check requests
2. Reporting to the watchdog when packages pass their explicit health
checks
3. Getting packages that support explicit health checks

Bug: 120598832
Test: WIP (When there is an implementation of
ExplicitHealthCheckService in ExtServices)
Change-Id: Ia030671c99699bd8d8273f32a97a1d3b7b015d3b