* Add third radio button to support cloud storage download preference
* Handle download preferences and UI states, while switching between three options
* Add method to check if provider is in use in Cloud Storage API
* Preferences cloud storage strings saved as resource url
* Updated old in-content with cloud storage pref UI changes
* Turn cloud storage API on/off using generic pref
MozReview-Commit-ID: AE3MlhgKp2C
* Has storage providers metadata
* Scan for storage providers returning preferred provider
* Helper methods to access cloud services prefs
* Helper method to set cloud storage as default download directory
* Notify observers for displaying cloud storage prompt
* Support pref value browser.downloads.folderList 3
* Read dropbox custom downloadpath from info.json
* Handle rejected providers by saving response in cloud.services.rejected.key
* Load providers metadata from providers.json
* Tested using add-on implemented in Bug 1365129
* Tests for Linux, Mac and Win Platform
MozReview-Commit-ID: LrmoDfsRTBV
