Mock getDeviceStorage so the gallery app works

This PR adds a `getDeviceStorage` API shim so the gallery works. It works with a predefined set of images that can be specified on the top (in base64 format, the only image in this PR is a blue square).

When viewing the gallery in Firefox, add a rule to the thumbnails that they have a fixed width and height because otherwise they have a 0x0 size in FF desktop. Have to see what that is in Gaia. Other than that it works awesome, detail view works and even the effects.