Bug 998847 - [Settings] refactor storage.js with AMD pattern

to replace https://github.com/mozilla-b2g/gaia/pull/19151/files
I was thinking to mark `enabled` of `AppStorage` as a private property. The property was kept for internal use where `AppStorage` may be able to decide whether to enable/disable itself (by observing if there are observers on its properties) in the future, so that it should not be exposed.