Preserve failure history when calling startObservingHealth

Instead of always creating a new MonitoredPackage every time
PackageWatchdog#startObservingHealth is called, just update
the duration of an existing MonitoredPackage if one exists. This
means that the failure history will be preserved.

Test: atest PackageWatchdogTest
Bug: 150114865
Change-Id: I6d6e3e0e893a603fda50df833bc5b6ce1757b6ec