Allow persistent observers to define packages they care about

If a persistent observer does not explicitly declare
that Package Watchdog should watch a package, it may
still start observing a failing package. Add a
method to the PackageHealthObserver interface that
allows an observer to define the conditions for which
it will start observing a package.

Test: n/a since no observers are currently persistent,
      current test pass
Bug: 136135457

Change-Id: I849d60e037248f0bbcd71b90b6c0cc3e6a68e5dc