Handle mitigation count logic in RescueParty

Rather than relying on a global rescue level property,
perform rescue logic based on the number of times
mitigation has been performed for a given package
already. This value is provided by PackageWatchdog.

Removed some logic that queried the state of the
rescue level sysprop. This property cannot be removed
yet, since the boot loop mitigation is unchanged for now.

Test: atest RescuePartyTest
Bug: 172206136
Change-Id: I90981ce202bd2649bcaf33882d9bcf7b4d1a2266