Updating IconCache to maintain a persistent of icons

> IconDB maintains a DB of icons keyed on ComponentName + User
> During loader, icons & labels are loaded first from the DB, and
if the entry doesn't exist, loaded using packageManager
> After the loader completes, IconDB updates any entry which might have
changed, while the launcher was dead.

Change-Id: I7a6021cb6d1ca1e66fa5a0bdd21e1543e0cf66fc