Fix NPE when clearing preferred packages.

Fixes issue #2507124:
cts.PackageManagerRequiringPermissionsTest:testClearPackagePreferredActivities
is failing with NPE

Also introduce some compatibility code to simple ignore calls to set
preferred packages from old applications, and changes some package manager
logs to Slog.

Change-Id: Id3f98ac8e106da526ba4c12879a9ddaabf28f462