Add boot loop de-escalation logic

Similar to work that has already been done for package failures,
pass a mitigation count to an observer interested in boot loops.
Since this logic is handled using sysprops, the mitigation count
will be reset after the default escalation window, rather than
decreasing with a sliding window.

Migrated elapsedRealTime to uptimeMillis in all instances in
PackageWatchdog. This should not have any impact, since the only
difference is that elapsedRealTime counted time in deep sleep,
which will not be an important factor for boot loops detection.

Test: atest PackageWatchdogTest
Test: atest RescuePartyTest
Bug: 172206136
Change-Id: I7e59f3fa32544bd410d8508e6529c77049a70df0