(into master) Bug 975211 - Create backend logic to provide list of Tiles and associated metadata (image, text, background color)
DirectoryTilesProvider with sample data and placeholder image
replace providerData.thumbUrl and placeholder.jpg by providerData.imageUri and embedded base64 encoded images
Copy over getLocale() code from toolkit/mozapps/extensions/AddonManager.jsm that wraps calls with try/catch.