Use a default value in case of an invalid parameter

Since startObservingHealth is called during boot, it is less desirable
to cause boot loops by an uncaught exception. We will fall back to
DEFAULT_OBSERVING_DURATION_MS when invalid durationMs is passed.

See b/140780361 for more details about the design decision.

Bug: 140780361
Test: atest PackageWatchdogTest
Change-Id: I2bcbecb2dc4c2448ef697001dd93aea5f50f9dbf