PackageWatchdog listens for NetworkStack failures

In addition to the NetworkStack app monitoring, have PackageWatchdog
register an observer to NetworkStackClient to receive severe failure
notifications, and attempt a rollback if available.

The callback is registered in onPackagesReady(), which is called in the
boot sequence just before starting the NetworkStack.

Test: installed new networkstack, killed it twice, observe rollback
Test: unit test in change on top
Bug: 133725814
Change-Id: I2cb4200b78c2482cacc4bfe2ace1581b869be512