Instead of periodically syncing requests with the same information,
only call into the ExplicitHealthCheckController when the set
of packages with pending health checks has changed, or a new observer
has been registered. Add tests to verify that duplicate calls are not made.
Test: atest PackageWatchdogTest#testSyncHealthCheckRequests
Test: atest NetworkStagedRollbackTest
Bug: 150114865
Bug: 146767850
Change-Id: I2926e9c7689e0ac9c4a142263ffd50a4747d016f
449b5853ba