Refactor thumbnail size calculation

* moves generic logic into ThumbnailSizeCalculator class
* moves UI-specific logic into interface class
* fixes bug where non-bucketed sizes were served on devices with
  non-standard pixel density
* fixes bug where bucketed size was compared to css size instead of screen size
  for resizing

Change-Id: I8ba3380b74fcc8fb0a6ecc3f3140627411851ad0
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/196
Merge "Refactor thumbnail size calculation"