Bug 985084 - Part 2: Properly report userDisabled in the API; r=Unfocused

The change in 0633949f1000 was buggy. If a new Addon instance were
created, it would always return the value stored in the database. This
meant that if you set userDisabled = false and called getAddonByID(),
userDisabled would likely be reported as true.

--HG--
extra : rebase_source : 02be7302b6e168b5d56cbf02de0cca4810ccd700
extra : amend_source : 4cdc73bfbc826abd594d6f8a2374672be834dba8