Unbind explicit health check controller when possible

Ensure that calls to sync requests with the explicit health
check controller are always sent if the list of packages
pending health checks is empty, so that the controller can
unbind. This will allow extservices to be killed by lmkd
on low memory devices.

Test: atest PackageWatchdogTest
Test: atest NetworkStagedRollbackTest
Test: check logcat to see that the service is unbound
Bug: 156323728
Change-Id: If615a337760b2057b962284bde8565b593d82a50