Improve [Alpha page styling experiment] Image at top

* Thumbnail width rounded to upper bound to try to avoid upscaling.
* Image height defaults to 360px. When loaded resizes to 16:9 aspect ratio.
* Image height will never grow higher than 360px.
* Background size improved to cover the whole image area.
  * Will upscale/downscale image to fit the smallest dimension (background cover)
* Wider than 16:9 images will get loaded bigger to avoid upscaling
* Refactor into smaller methods and extract class for the actual Images
* Load images sequentially instead of all at once and try to pick the first one
instead of the first to load (images are sorted from better to worse)

Change-Id: Ia3989511d89f127b274a436b1064392e82cba1ba
Merge "Improve [Alpha page styling experiment] Image at top"