Add per-package escalation logic

Track the number of times a call has been made to an
observer to mitigate each MonitoredPackage object. This
"mitigation count" will be used as a proxy for determining
what rescue level to perform in RescueParty. A sliding
window is used so that this mitigation count may
de-escalate. The default value of this sliding window
is one hour.

A follow-up CL will integrate RescueParty's rescue level
mechanism with this logic.

Test: atest PackageWatchdogTest
Bug: 172206136
Change-Id: Idb97901ad1c8acbee15417ea35d29e67e9d4562e