Integrate Rescue Party with Package Watchdog

Integrate Rescue Party as an observer for Package
Watchdog, for managing package failures. Rescue Party
will be a persistent observer, meaning it may receive
failure calls for packages it has not explicitly asked
to observe.

Remove app failure calls and thresholding logic from
Rescue Party. Remove obsolete Rescue Party tests
and add persistent observer tests to
PackageWatchdogTest.

Test: atest PackageWatchdogTest
Test: atest RescuePartyTest
Test: atest StagedRollbackTest
Bug: 136135457
Change-Id: I55ec0de48acd5434255811feba758d38c9304478