Update Javadoc for PackageListObserver

PackageListObserver is created to resolve the broadcast delay for
the system service. It is migrated off the broadcast to use callback.
The in-process binder calls are just direct method calls. This change
update the PackageListObserver javadoc to avoid new user using it.

Bug: 290955674
Test: build pass. No code logic change just javadoc update.
Change-Id: I1f83d749d9b2f660580c3987ee45d285f4f744e7