Register and unregister might be out of order (5/n)

Calling unregisterHealthObserver() right after startObservingHealth()
will fail to unregister observers correctly because startObservingHealth()
do registration asynchronously.

Let's do unregistration asynchronously as well to keep things in order.

Bug: 178675924
Test: atest PackageWatchdogTest
Change-Id: I7dfea957fb88f0346cdeea20e626a728ec1a1325