Sync requests only when there are changes to report

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