ShortcutManager: rescan all apps after next OTA

- Normally when a user is unlocked, SM scans all updated apps
since the last scan to see if their manifest shortcuts have changed.

- There was a known issue that's been fixed already that'd result
in all shortcuts being removed.

- To work around this issue, let's scan all packages after the next
user unlock.

- This can be done by just renaming the XML attribute name to persist
the last app scan time.

Bug 30591723

Change-Id: Ie9f723ef30a1c991990d981193841cb203c094ad