Fix PackageWatchdog and add PackageWatchdogTest

Fixes:
1. Remove registered observer when removed from persisted file
2. Only call external observers after threshold is exceeded
3. Handle edge case where we reschedule package cleanup and elapsed time
is longer than scheduled duration
4. Modify code to allow easier testing

Bug: 120598832
Test: atest PackageWatchdogTest

Change-Id: I92181136fb5994a4d8ebe976be3138f210e853a5