Fix testExplicitHealthCheckFailureBeforeExpiry

The code doesn't work as intended. What we should do is:
1. set up so that health check duration is shorter than observation duration
2. move time forward so we fail the health check
3. check observer.mMitigatedPackages contains only APP_A
4. move time forward again to expire the observation duration
5. check APP_A is not notified again as a failed package

Also add a similar test where the observation duration is shorter than
the health check duration.

Bug: 141518951
Test: atest PackageWatchdogTest
Change-Id: Iba1cdc4fab8608982b416cdb463ed4b38d355c9f