When inserting directly into LauncherProvider, ensure item id and widget id are valid

-> We generate an item id, and allocate and bind a widget id if necessary when items
   are added to the LauncherProvider externally. Writing to the provider is protected
   by system|signature.

issue 16191557

Change-Id: I09472e41ad4c8302901a5e49b5817247e9b86695