Fix infinite loop during package-usage.list file upgrade

When upgrading from version 0 to version 1 of the file
'/data/system/package-usage.list', the PackageManagerService can get
stuck in an infinite loop if one of the listed packages does not
exist, e.g. because it had been uninstalled. Fix the issue by
refactoring the loop.

Bug: 28409278
Change-Id: Ia312bd0d04f696240445b710dd6a68b93c5d5946