Use unique preference key.

The UI code was wrongly using UID as the key - it is not
unique - resulting in keeping only one app per UID in the
UI and not showing permission controls for the other apps
in this UID. This change uses the unique package plus UID
as the preference key.

bug:23937944

Change-Id: I859afcf3ec8efdcb62d0e9dacd19f771436da084