ShortcutManager: Optimize package scanning

SM needs to check all packages when a user is unlocked in case
any apps have been upgraded while the user was not running,
in which case it'll publish the manifest shortcuts.

Currently it actually scans all apps' manifests.

This CL optimizes it by remembering the last scan time (for each user)
and checks only the packages that have been updated since then.

Bug 29069154

Change-Id: Id10b6be8915fe7c1e26daccde7951ddbd01ea452